diff --git a/deepspeech/decoders/README.md b/deepspeech/decoders/README.md
new file mode 100644
index 000000000..11eb56063
--- /dev/null
+++ b/deepspeech/decoders/README.md
@@ -0,0 +1,13 @@
+# Decoders
+
+## Reference
+### CTC Prefix Beam Search
+* [Sequence Modeling With CTC](https://distill.pub/2017/ctc/)
+* [First-Pass Large Vocabulary Continuous Speech Recognition using Bi-Directional Recurrent DNNs](https://arxiv.org/pdf/1408.2873.pdf)
+
+### CTC Prefix Score & Join CTC/ATT One-passing Decoding
+* [Hybrid CTC/Attention Architecture for End-to-End Speech Recognition](http://www.ifp.illinois.edu/speech/speech_web_lg/slides/2019/watanabe_hybridCTCAttention_2017.pdf)
+* [Vectorized Beam Search for CTC-Attention-based Speech Recognition](https://www.isca-speech.org/archive/pdfs/interspeech_2019/seki19b_interspeech.pdf)
+
+### Streaming Join CTC/ATT Beam Search
+* [STREAMING TRANSFORMER ASR WITH BLOCKWISE SYNCHRONOUS BEAM SEARCH](https://arxiv.org/abs/2006.14941)
\ No newline at end of file
diff --git a/deepspeech/decoders/scores/__init__.py b/deepspeech/decoders/scores/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/deepspeech/decoders/scores/ctc.py b/deepspeech/decoders/scores/ctc.py
new file mode 100644
index 000000000..6e5a4c538
--- /dev/null
+++ b/deepspeech/decoders/scores/ctc.py
@@ -0,0 +1,158 @@
+"""ScorerInterface implementation for CTC."""
+
+import numpy as np
+import paddle
+
+from .ctc_prefix_score import CTCPrefixScore
+from .ctc_prefix_score import CTCPrefixScoreTH
+from .scorer_interface import BatchPartialScorerInterface
+
+
+class CTCPrefixScorer(BatchPartialScorerInterface):
+    """Decoder interface wrapper for CTCPrefixScore."""
+
+    def __init__(self, ctc: paddle.nn.Layer, eos: int):
+        """Initialize class.
+
+        Args:
+            ctc (paddle.nn.Layer): The CTC implementation.
+                For example, :class:`deepspeech.modules.ctc.CTC`
+            eos (int): The end-of-sequence id.
+
+        """
+        self.ctc = ctc
+        self.eos = eos
+        self.impl = None
+
+    def init_state(self, x: paddle.Tensor):
+        """Get an initial state for decoding.
+
+        Args:
+            x (paddle.Tensor): The encoded feature tensor
+
+        Returns: initial state
+
+        """
+        logp = self.ctc.log_softmax(x.unsqueeze(0)).squeeze(0).numpy()
+        # TODO(karita): use CTCPrefixScoreTH
+        self.impl = CTCPrefixScore(logp, 0, self.eos, np)
+        return 0, self.impl.initial_state()
+
+    def select_state(self, state, i, new_id=None):
+        """Select state with relative ids in the main beam search.
+
+        Args:
+            state: Decoder state for prefix tokens
+            i (int): Index to select a state in the main beam search
+            new_id (int): New label id to select a state if necessary
+
+        Returns:
+            state: pruned state
+
+        """
+        if type(state) == tuple:
+            if len(state) == 2:  # for CTCPrefixScore
+                sc, st = state
+                return sc[i], st[i]
+            else:  # for CTCPrefixScoreTH (need new_id > 0)
+                r, log_psi, f_min, f_max, scoring_idmap = state
+                s = log_psi[i, new_id].expand(log_psi.size(1))
+                if scoring_idmap is not None:
+                    return r[:, :, i, scoring_idmap[i, new_id]], s, f_min, f_max
+                else:
+                    return r[:, :, i, new_id], s, f_min, f_max
+        return None if state is None else state[i]
+
+    def score_partial(self, y, ids, state, x):
+        """Score new token.
+
+        Args:
+            y (paddle.Tensor): 1D prefix token
+            next_tokens (paddle.Tensor): paddle.int64 next token to score
+            state: decoder state for prefix tokens
+            x (paddle.Tensor): 2D encoder feature that generates ys
+
+        Returns:
+            tuple[paddle.Tensor, Any]:
+                Tuple of a score tensor for y that has a shape `(len(next_tokens),)`
+                and next state for ys
+
+        """
+        prev_score, state = state
+        presub_score, new_st = self.impl(y.cpu(), ids.cpu(), state)
+        tscore = paddle.to_tensor(
+            presub_score - prev_score, place=x.place, dtype=x.dtype
+        )
+        return tscore, (presub_score, new_st)
+
+    def batch_init_state(self, x: paddle.Tensor):
+        """Get an initial state for decoding.
+
+        Args:
+            x (paddle.Tensor): The encoded feature tensor
+
+        Returns: initial state
+
+        """
+        logp = self.ctc.log_softmax(x.unsqueeze(0))  # assuming batch_size = 1
+        xlen = paddle.to_tensor([logp.size(1)])
+        self.impl = CTCPrefixScoreTH(logp, xlen, 0, self.eos)
+        return None
+
+    def batch_score_partial(self, y, ids, state, x):
+        """Score new token.
+
+        Args:
+            y (paddle.Tensor): 1D prefix token
+            ids (paddle.Tensor): paddle.int64 next token to score
+            state: decoder state for prefix tokens
+            x (paddle.Tensor): 2D encoder feature that generates ys
+
+        Returns:
+            tuple[paddle.Tensor, Any]:
+                Tuple of a score tensor for y that has a shape `(len(next_tokens),)`
+                and next state for ys
+
+        """
+        batch_state = (
+            (
+                paddle.stack([s[0] for s in state], axis=2),
+                paddle.stack([s[1] for s in state]),
+                state[0][2],
+                state[0][3],
+            )
+            if state[0] is not None
+            else None
+        )
+        return self.impl(y, batch_state, ids)
+
+    def extend_prob(self, x: paddle.Tensor):
+        """Extend probs for decoding.
+
+        This extension is for streaming decoding
+        as in Eq (14) in https://arxiv.org/abs/2006.14941
+
+        Args:
+            x (paddle.Tensor): The encoded feature tensor
+
+        """
+        logp = self.ctc.log_softmax(x.unsqueeze(0))
+        self.impl.extend_prob(logp)
+
+    def extend_state(self, state):
+        """Extend state for decoding.
+
+        This extension is for streaming decoding
+        as in Eq (14) in https://arxiv.org/abs/2006.14941
+
+        Args:
+            state: The states of hyps
+
+        Returns: exteded state
+
+        """
+        new_state = []
+        for s in state:
+            new_state.append(self.impl.extend_state(s))
+
+        return new_state
diff --git a/deepspeech/decoders/scores/ctc_prefix_score.py b/deepspeech/decoders/scores/ctc_prefix_score.py
new file mode 100644
index 000000000..2ca00ebf0
--- /dev/null
+++ b/deepspeech/decoders/scores/ctc_prefix_score.py
@@ -0,0 +1,359 @@
+#!/usr/bin/env python3
+
+# Copyright 2018 Mitsubishi Electric Research Labs (Takaaki Hori)
+#  Apache 2.0  (http://www.apache.org/licenses/LICENSE-2.0)
+
+import torch
+
+import numpy as np
+import six
+
+
+class CTCPrefixScoreTH():
+    """Batch processing of CTCPrefixScore
+
+    which is based on Algorithm 2 in WATANABE et al.
+    "HYBRID CTC/ATTENTION ARCHITECTURE FOR END-TO-END SPEECH RECOGNITION,"
+    but extended to efficiently compute the label probablities for multiple
+    hypotheses simultaneously
+    See also Seki et al. "Vectorized Beam Search for CTC-Attention-Based
+    Speech Recognition," In INTERSPEECH (pp. 3825-3829), 2019.
+    """
+
+    def __init__(self, x, xlens, blank, eos, margin=0):
+        """Construct CTC prefix scorer
+
+        :param torch.Tensor x: input label posterior sequences (B, T, O)
+        :param torch.Tensor xlens: input lengths (B,)
+        :param int blank: blank label id
+        :param int eos: end-of-sequence id
+        :param int margin: margin parameter for windowing (0 means no windowing)
+        """
+        # In the comment lines,
+        # we assume T: input_length, B: batch size, W: beam width, O: output dim.
+        self.logzero = -10000000000.0
+        self.blank = blank
+        self.eos = eos
+        self.batch = x.size(0)
+        self.input_length = x.size(1)
+        self.odim = x.size(2)
+        self.dtype = x.dtype
+        self.device = (
+            torch.device("cuda:%d" % x.get_device())
+            if x.is_cuda
+            else torch.device("cpu")
+        )
+        # Pad the rest of posteriors in the batch
+        # TODO(takaaki-hori): need a better way without for-loops
+        for i, l in enumerate(xlens):
+            if l < self.input_length:
+                x[i, l:, :] = self.logzero
+                x[i, l:, blank] = 0
+        # Reshape input x
+        xn = x.transpose(0, 1)  # (B, T, O) -> (T, B, O)
+        xb = xn[:, :, self.blank].unsqueeze(2).expand(-1, -1, self.odim)
+        self.x = torch.stack([xn, xb])  # (2, T, B, O)
+        self.end_frames = torch.as_tensor(xlens) - 1
+
+        # Setup CTC windowing
+        self.margin = margin
+        if margin > 0:
+            self.frame_ids = torch.arange(
+                self.input_length, dtype=self.dtype, device=self.device
+            )
+        # Base indices for index conversion
+        self.idx_bh = None
+        self.idx_b = torch.arange(self.batch, device=self.device)
+        self.idx_bo = (self.idx_b * self.odim).unsqueeze(1)
+
+    def __call__(self, y, state, scoring_ids=None, att_w=None):
+        """Compute CTC prefix scores for next labels
+
+        :param list y: prefix label sequences
+        :param tuple state: previous CTC state
+        :param torch.Tensor pre_scores: scores for pre-selection of hypotheses (BW, O)
+        :param torch.Tensor att_w: attention weights to decide CTC window
+        :return new_state, ctc_local_scores (BW, O)
+        """
+        output_length = len(y[0]) - 1  # ignore sos
+        last_ids = [yi[-1] for yi in y]  # last output label ids
+        n_bh = len(last_ids)  # batch * hyps
+        n_hyps = n_bh // self.batch  # assuming each utterance has the same # of hyps
+        self.scoring_num = scoring_ids.size(-1) if scoring_ids is not None else 0
+        # prepare state info
+        if state is None:
+            r_prev = torch.full(
+                (self.input_length, 2, self.batch, n_hyps),
+                self.logzero,
+                dtype=self.dtype,
+                device=self.device,
+            )
+            r_prev[:, 1] = torch.cumsum(self.x[0, :, :, self.blank], 0).unsqueeze(2)
+            r_prev = r_prev.view(-1, 2, n_bh)
+            s_prev = 0.0
+            f_min_prev = 0
+            f_max_prev = 1
+        else:
+            r_prev, s_prev, f_min_prev, f_max_prev = state
+
+        # select input dimensions for scoring
+        if self.scoring_num > 0:
+            scoring_idmap = torch.full(
+                (n_bh, self.odim), -1, dtype=torch.long, device=self.device
+            )
+            snum = self.scoring_num
+            if self.idx_bh is None or n_bh > len(self.idx_bh):
+                self.idx_bh = torch.arange(n_bh, device=self.device).view(-1, 1)
+            scoring_idmap[self.idx_bh[:n_bh], scoring_ids] = torch.arange(
+                snum, device=self.device
+            )
+            scoring_idx = (
+                scoring_ids + self.idx_bo.repeat(1, n_hyps).view(-1, 1)
+            ).view(-1)
+            x_ = torch.index_select(
+                self.x.view(2, -1, self.batch * self.odim), 2, scoring_idx
+            ).view(2, -1, n_bh, snum)
+        else:
+            scoring_ids = None
+            scoring_idmap = None
+            snum = self.odim
+            x_ = self.x.unsqueeze(3).repeat(1, 1, 1, n_hyps, 1).view(2, -1, n_bh, snum)
+
+        # new CTC forward probs are prepared as a (T x 2 x BW x S) tensor
+        # that corresponds to r_t^n(h) and r_t^b(h) in a batch.
+        r = torch.full(
+            (self.input_length, 2, n_bh, snum),
+            self.logzero,
+            dtype=self.dtype,
+            device=self.device,
+        )
+        if output_length == 0:
+            r[0, 0] = x_[0, 0]
+
+        r_sum = torch.logsumexp(r_prev, 1)
+        log_phi = r_sum.unsqueeze(2).repeat(1, 1, snum)
+        if scoring_ids is not None:
+            for idx in range(n_bh):
+                pos = scoring_idmap[idx, last_ids[idx]]
+                if pos >= 0:
+                    log_phi[:, idx, pos] = r_prev[:, 1, idx]
+        else:
+            for idx in range(n_bh):
+                log_phi[:, idx, last_ids[idx]] = r_prev[:, 1, idx]
+
+        # decide start and end frames based on attention weights
+        if att_w is not None and self.margin > 0:
+            f_arg = torch.matmul(att_w, self.frame_ids)
+            f_min = max(int(f_arg.min().cpu()), f_min_prev)
+            f_max = max(int(f_arg.max().cpu()), f_max_prev)
+            start = min(f_max_prev, max(f_min - self.margin, output_length, 1))
+            end = min(f_max + self.margin, self.input_length)
+        else:
+            f_min = f_max = 0
+            start = max(output_length, 1)
+            end = self.input_length
+
+        # compute forward probabilities log(r_t^n(h)) and log(r_t^b(h))
+        for t in range(start, end):
+            rp = r[t - 1]
+            rr = torch.stack([rp[0], log_phi[t - 1], rp[0], rp[1]]).view(
+                2, 2, n_bh, snum
+            )
+            r[t] = torch.logsumexp(rr, 1) + x_[:, t]
+
+        # compute log prefix probabilities log(psi)
+        log_phi_x = torch.cat((log_phi[0].unsqueeze(0), log_phi[:-1]), dim=0) + x_[0]
+        if scoring_ids is not None:
+            log_psi = torch.full(
+                (n_bh, self.odim), self.logzero, dtype=self.dtype, device=self.device
+            )
+            log_psi_ = torch.logsumexp(
+                torch.cat((log_phi_x[start:end], r[start - 1, 0].unsqueeze(0)), dim=0),
+                dim=0,
+            )
+            for si in range(n_bh):
+                log_psi[si, scoring_ids[si]] = log_psi_[si]
+        else:
+            log_psi = torch.logsumexp(
+                torch.cat((log_phi_x[start:end], r[start - 1, 0].unsqueeze(0)), dim=0),
+                dim=0,
+            )
+
+        for si in range(n_bh):
+            log_psi[si, self.eos] = r_sum[self.end_frames[si // n_hyps], si]
+
+        # exclude blank probs
+        log_psi[:, self.blank] = self.logzero
+
+        return (log_psi - s_prev), (r, log_psi, f_min, f_max, scoring_idmap)
+
+    def index_select_state(self, state, best_ids):
+        """Select CTC states according to best ids
+
+        :param state    : CTC state
+        :param best_ids : index numbers selected by beam pruning (B, W)
+        :return selected_state
+        """
+        r, s, f_min, f_max, scoring_idmap = state
+        # convert ids to BHO space
+        n_bh = len(s)
+        n_hyps = n_bh // self.batch
+        vidx = (best_ids + (self.idx_b * (n_hyps * self.odim)).view(-1, 1)).view(-1)
+        # select hypothesis scores
+        s_new = torch.index_select(s.view(-1), 0, vidx)
+        s_new = s_new.view(-1, 1).repeat(1, self.odim).view(n_bh, self.odim)
+        # convert ids to BHS space (S: scoring_num)
+        if scoring_idmap is not None:
+            snum = self.scoring_num
+            hyp_idx = (best_ids // self.odim + (self.idx_b * n_hyps).view(-1, 1)).view(
+                -1
+            )
+            label_ids = torch.fmod(best_ids, self.odim).view(-1)
+            score_idx = scoring_idmap[hyp_idx, label_ids]
+            score_idx[score_idx == -1] = 0
+            vidx = score_idx + hyp_idx * snum
+        else:
+            snum = self.odim
+        # select forward probabilities
+        r_new = torch.index_select(r.view(-1, 2, n_bh * snum), 2, vidx).view(
+            -1, 2, n_bh
+        )
+        return r_new, s_new, f_min, f_max
+
+    def extend_prob(self, x):
+        """Extend CTC prob.
+
+        :param torch.Tensor x: input label posterior sequences (B, T, O)
+        """
+
+        if self.x.shape[1] < x.shape[1]:  # self.x (2,T,B,O); x (B,T,O)
+            # Pad the rest of posteriors in the batch
+            # TODO(takaaki-hori): need a better way without for-loops
+            xlens = [x.size(1)]
+            for i, l in enumerate(xlens):
+                if l < self.input_length:
+                    x[i, l:, :] = self.logzero
+                    x[i, l:, self.blank] = 0
+            tmp_x = self.x
+            xn = x.transpose(0, 1)  # (B, T, O) -> (T, B, O)
+            xb = xn[:, :, self.blank].unsqueeze(2).expand(-1, -1, self.odim)
+            self.x = torch.stack([xn, xb])  # (2, T, B, O)
+            self.x[:, : tmp_x.shape[1], :, :] = tmp_x
+            self.input_length = x.size(1)
+            self.end_frames = torch.as_tensor(xlens) - 1
+
+    def extend_state(self, state):
+        """Compute CTC prefix state.
+
+
+        :param state    : CTC state
+        :return ctc_state
+        """
+
+        if state is None:
+            # nothing to do
+            return state
+        else:
+            r_prev, s_prev, f_min_prev, f_max_prev = state
+
+            r_prev_new = torch.full(
+                (self.input_length, 2),
+                self.logzero,
+                dtype=self.dtype,
+                device=self.device,
+            )
+            start = max(r_prev.shape[0], 1)
+            r_prev_new[0:start] = r_prev
+            for t in six.moves.range(start, self.input_length):
+                r_prev_new[t, 1] = r_prev_new[t - 1, 1] + self.x[0, t, :, self.blank]
+
+            return (r_prev_new, s_prev, f_min_prev, f_max_prev)
+
+
+class CTCPrefixScore():
+    """Compute CTC label sequence scores
+
+    which is based on Algorithm 2 in WATANABE et al.
+    "HYBRID CTC/ATTENTION ARCHITECTURE FOR END-TO-END SPEECH RECOGNITION,"
+    but extended to efficiently compute the probablities of multiple labels
+    simultaneously
+    """
+
+    def __init__(self, x, blank, eos, xp):
+        self.xp = xp
+        self.logzero = -10000000000.0
+        self.blank = blank
+        self.eos = eos
+        self.input_length = len(x)
+        self.x = x
+
+    def initial_state(self):
+        """Obtain an initial CTC state
+
+        :return: CTC state
+        """
+        # initial CTC state is made of a frame x 2 tensor that corresponds to
+        # r_t^n(<sos>) and r_t^b(<sos>), where 0 and 1 of axis=1 represent
+        # superscripts n and b (non-blank and blank), respectively.
+        r = self.xp.full((self.input_length, 2), self.logzero, dtype=np.float32)
+        r[0, 1] = self.x[0, self.blank]
+        for i in six.moves.range(1, self.input_length):
+            r[i, 1] = r[i - 1, 1] + self.x[i, self.blank]
+        return r
+
+    def __call__(self, y, cs, r_prev):
+        """Compute CTC prefix scores for next labels
+
+        :param y     : prefix label sequence
+        :param cs    : array of next labels
+        :param r_prev: previous CTC state
+        :return ctc_scores, ctc_states
+        """
+        # initialize CTC states
+        output_length = len(y) - 1  # ignore sos
+        # new CTC states are prepared as a frame x (n or b) x n_labels tensor
+        # that corresponds to r_t^n(h) and r_t^b(h).
+        r = self.xp.ndarray((self.input_length, 2, len(cs)), dtype=np.float32)
+        xs = self.x[:, cs]
+        if output_length == 0:
+            r[0, 0] = xs[0]
+            r[0, 1] = self.logzero
+        else:
+            r[output_length - 1] = self.logzero
+
+        # prepare forward probabilities for the last label
+        r_sum = self.xp.logaddexp(
+            r_prev[:, 0], r_prev[:, 1]
+        )  # log(r_t^n(g) + r_t^b(g))
+        last = y[-1]
+        if output_length > 0 and last in cs:
+            log_phi = self.xp.ndarray((self.input_length, len(cs)), dtype=np.float32)
+            for i in six.moves.range(len(cs)):
+                log_phi[:, i] = r_sum if cs[i] != last else r_prev[:, 1]
+        else:
+            log_phi = r_sum
+
+        # compute forward probabilities log(r_t^n(h)), log(r_t^b(h)),
+        # and log prefix probabilities log(psi)
+        start = max(output_length, 1)
+        log_psi = r[start - 1, 0]
+        for t in six.moves.range(start, self.input_length):
+            r[t, 0] = self.xp.logaddexp(r[t - 1, 0], log_phi[t - 1]) + xs[t]
+            r[t, 1] = (
+                self.xp.logaddexp(r[t - 1, 0], r[t - 1, 1]) + self.x[t, self.blank]
+            )
+            log_psi = self.xp.logaddexp(log_psi, log_phi[t - 1] + xs[t])
+
+        # get P(...eos|X) that ends with the prefix itself
+        eos_pos = self.xp.where(cs == self.eos)[0]
+        if len(eos_pos) > 0:
+            log_psi[eos_pos] = r_sum[-1]  # log(r_T^n(g) + r_T^b(g))
+
+        # exclude blank probs
+        blank_pos = self.xp.where(cs == self.blank)[0]
+        if len(blank_pos) > 0:
+            log_psi[blank_pos] = self.logzero
+
+        # return the log prefix probability and CTC states, where the label axis
+        # of the CTC states is moved to the first axis to slice it easily
+        return log_psi, self.xp.rollaxis(r, 2)
diff --git a/deepspeech/decoders/scores/length_bonus.py b/deepspeech/decoders/scores/length_bonus.py
new file mode 100644
index 000000000..76f45f801
--- /dev/null
+++ b/deepspeech/decoders/scores/length_bonus.py
@@ -0,0 +1,61 @@
+"""Length bonus module."""
+from typing import Any
+from typing import List
+from typing import Tuple
+
+import paddle
+
+from .scorer_interface import BatchScorerInterface
+
+
+class LengthBonus(BatchScorerInterface):
+    """Length bonus in beam search."""
+
+    def __init__(self, n_vocab: int):
+        """Initialize class.
+
+        Args:
+            n_vocab (int): The number of tokens in vocabulary for beam search
+
+        """
+        self.n = n_vocab
+
+    def score(self, y, state, x):
+        """Score new token.
+
+        Args:
+            y (paddle.Tensor): 1D paddle.int64 prefix tokens.
+            state: Scorer state for prefix tokens
+            x (paddle.Tensor): 2D encoder feature that generates ys.
+
+        Returns:
+            tuple[paddle.Tensor, Any]: Tuple of
+                paddle.float32 scores for next token (n_vocab)
+                and None
+
+        """
+        return paddle.to_tensor([1.0], place=x.place, dtype=x.dtype).expand(self.n), None
+
+    def batch_score(
+        self, ys: paddle.Tensor, states: List[Any], xs: paddle.Tensor
+    ) -> Tuple[paddle.Tensor, List[Any]]:
+        """Score new token batch.
+
+        Args:
+            ys (paddle.Tensor): paddle.int64 prefix tokens (n_batch, ylen).
+            states (List[Any]): Scorer states for prefix tokens.
+            xs (paddle.Tensor):
+                The encoder feature that generates ys (n_batch, xlen, n_feat).
+
+        Returns:
+            tuple[paddle.Tensor, List[Any]]: Tuple of
+                batchfied scores for next token with shape of `(n_batch, n_vocab)`
+                and next state list for ys.
+
+        """
+        return (
+            paddle.to_tensor([1.0], place=xs.place, dtype=xs.dtype).expand(
+                ys.shape[0], self.n
+            ),
+            None,
+        )
diff --git a/deepspeech/decoders/scores/ngram.py b/deepspeech/decoders/scores/ngram.py
new file mode 100644
index 000000000..9809427bd
--- /dev/null
+++ b/deepspeech/decoders/scores/ngram.py
@@ -0,0 +1,102 @@
+"""Ngram lm implement."""
+
+from abc import ABC
+
+import kenlm
+import paddle
+
+from .scorer_interface import BatchScorerInterface
+from .scorer_interface import PartialScorerInterface
+
+
+class Ngrambase(ABC):
+    """Ngram base implemented through ScorerInterface."""
+
+    def __init__(self, ngram_model, token_list):
+        """Initialize Ngrambase.
+
+        Args:
+            ngram_model: ngram model path
+            token_list: token list from dict or model.json
+
+        """
+        self.chardict = [x if x != "<eos>" else "</s>" for x in token_list]
+        self.charlen = len(self.chardict)
+        self.lm = kenlm.LanguageModel(ngram_model)
+        self.tmpkenlmstate = kenlm.State()
+
+    def init_state(self, x):
+        """Initialize tmp state."""
+        state = kenlm.State()
+        self.lm.NullContextWrite(state)
+        return state
+
+    def score_partial_(self, y, next_token, state, x):
+        """Score interface for both full and partial scorer.
+
+        Args:
+            y: previous char
+            next_token: next token need to be score
+            state: previous state
+            x: encoded feature
+
+        Returns:
+            tuple[paddle.Tensor, List[Any]]: Tuple of
+                batchfied scores for next token with shape of `(n_batch, n_vocab)`
+                and next state list for ys.
+
+        """
+        out_state = kenlm.State()
+        ys = self.chardict[y[-1]] if y.shape[0] > 1 else "<s>"
+        self.lm.BaseScore(state, ys, out_state)
+        scores = paddle.empty_like(next_token, dtype=x.dtype)
+        for i, j in enumerate(next_token):
+            scores[i] = self.lm.BaseScore(
+                out_state, self.chardict[j], self.tmpkenlmstate
+            )
+        return scores, out_state
+
+
+class NgramFullScorer(Ngrambase, BatchScorerInterface):
+    """Fullscorer for ngram."""
+
+    def score(self, y, state, x):
+        """Score interface for both full and partial scorer.
+
+        Args:
+            y: previous char
+            state: previous state
+            x: encoded feature
+
+        Returns:
+            tuple[paddle.Tensor, List[Any]]: Tuple of
+                batchfied scores for next token with shape of `(n_batch, n_vocab)`
+                and next state list for ys.
+
+        """
+        return self.score_partial_(y, paddle.to_tensor(range(self.charlen)), state, x)
+
+
+class NgramPartScorer(Ngrambase, PartialScorerInterface):
+    """Partialscorer for ngram."""
+
+    def score_partial(self, y, next_token, state, x):
+        """Score interface for both full and partial scorer.
+
+        Args:
+            y: previous char
+            next_token: next token need to be score
+            state: previous state
+            x: encoded feature
+
+        Returns:
+            tuple[paddle.Tensor, List[Any]]: Tuple of
+                batchfied scores for next token with shape of `(n_batch, n_vocab)`
+                and next state list for ys.
+
+        """
+        return self.score_partial_(y, next_token, state, x)
+
+    def select_state(self, state, i):
+        """Empty select state for scorer interface."""
+        return state
diff --git a/deepspeech/decoders/scores/score_interface.py b/deepspeech/decoders/scores/score_interface.py
new file mode 100644
index 000000000..c52f8d190
--- /dev/null
+++ b/deepspeech/decoders/scores/score_interface.py
@@ -0,0 +1,188 @@
+"""Scorer interface module."""
+
+from typing import Any
+from typing import List
+from typing import Tuple
+
+import paddle
+import warnings
+
+
+class ScorerInterface:
+    """Scorer interface for beam search.
+
+    The scorer performs scoring of the all tokens in vocabulary.
+
+    Examples:
+        * Search heuristics
+            * :class:`scorers.length_bonus.LengthBonus`
+        * Decoder networks of the sequence-to-sequence models
+            * :class:`transformer.decoder.Decoder`
+            * :class:`rnn.decoders.Decoder`
+        * Neural language models
+            * :class:`lm.transformer.TransformerLM`
+            * :class:`lm.default.DefaultRNNLM`
+            * :class:`lm.seq_rnn.SequentialRNNLM`
+
+    """
+
+    def init_state(self, x: paddle.Tensor) -> Any:
+        """Get an initial state for decoding (optional).
+
+        Args:
+            x (paddle.Tensor): The encoded feature tensor
+
+        Returns: initial state
+
+        """
+        return None
+
+    def select_state(self, state: Any, i: int, new_id: int = None) -> Any:
+        """Select state with relative ids in the main beam search.
+
+        Args:
+            state: Decoder state for prefix tokens
+            i (int): Index to select a state in the main beam search
+            new_id (int): New label index to select a state if necessary
+
+        Returns:
+            state: pruned state
+
+        """
+        return None if state is None else state[i]
+
+    def score(
+        self, y: paddle.Tensor, state: Any, x: paddle.Tensor
+    ) -> Tuple[paddle.Tensor, Any]:
+        """Score new token (required).
+
+        Args:
+            y (paddle.Tensor): 1D paddle.int64 prefix tokens.
+            state: Scorer state for prefix tokens
+            x (paddle.Tensor): The encoder feature that generates ys.
+
+        Returns:
+            tuple[paddle.Tensor, Any]: Tuple of
+                scores for next token that has a shape of `(n_vocab)`
+                and next state for ys
+
+        """
+        raise NotImplementedError
+
+    def final_score(self, state: Any) -> float:
+        """Score eos (optional).
+
+        Args:
+            state: Scorer state for prefix tokens
+
+        Returns:
+            float: final score
+
+        """
+        return 0.0
+
+
+class BatchScorerInterface(ScorerInterface):
+    """Batch scorer interface."""
+
+    def batch_init_state(self, x: paddle.Tensor) -> Any:
+        """Get an initial state for decoding (optional).
+
+        Args:
+            x (paddle.Tensor): The encoded feature tensor
+
+        Returns: initial state
+
+        """
+        return self.init_state(x)
+
+    def batch_score(
+        self, ys: paddle.Tensor, states: List[Any], xs: paddle.Tensor
+    ) -> Tuple[paddle.Tensor, List[Any]]:
+        """Score new token batch (required).
+
+        Args:
+            ys (paddle.Tensor): paddle.int64 prefix tokens (n_batch, ylen).
+            states (List[Any]): Scorer states for prefix tokens.
+            xs (paddle.Tensor):
+                The encoder feature that generates ys (n_batch, xlen, n_feat).
+
+        Returns:
+            tuple[paddle.Tensor, List[Any]]: Tuple of
+                batchfied scores for next token with shape of `(n_batch, n_vocab)`
+                and next state list for ys.
+
+        """
+        warnings.warn(
+            "{} batch score is implemented through for loop not parallelized".format(
+                self.__class__.__name__
+            )
+        )
+        scores = list()
+        outstates = list()
+        for i, (y, state, x) in enumerate(zip(ys, states, xs)):
+            score, outstate = self.score(y, state, x)
+            outstates.append(outstate)
+            scores.append(score)
+        scores = paddle.cat(scores, 0).view(ys.shape[0], -1)
+        return scores, outstates
+
+
+class PartialScorerInterface(ScorerInterface):
+    """Partial scorer interface for beam search.
+
+    The partial scorer performs scoring when non-partial scorer finished scoring,
+    and receives pre-pruned next tokens to score because it is too heavy to score
+    all the tokens.
+
+    Examples:
+         * Prefix search for connectionist-temporal-classification models
+             * :class:`espnet.nets.scorers.ctc.CTCPrefixScorer`
+
+    """
+
+    def score_partial(
+        self, y: paddle.Tensor, next_tokens: paddle.Tensor, state: Any, x: paddle.Tensor
+    ) -> Tuple[paddle.Tensor, Any]:
+        """Score new token (required).
+
+        Args:
+            y (paddle.Tensor): 1D prefix token
+            next_tokens (paddle.Tensor): paddle.int64 next token to score
+            state: decoder state for prefix tokens
+            x (paddle.Tensor): The encoder feature that generates ys
+
+        Returns:
+            tuple[paddle.Tensor, Any]:
+                Tuple of a score tensor for y that has a shape `(len(next_tokens),)`
+                and next state for ys
+
+        """
+        raise NotImplementedError
+
+
+class BatchPartialScorerInterface(BatchScorerInterface, PartialScorerInterface):
+    """Batch partial scorer interface for beam search."""
+
+    def batch_score_partial(
+        self,
+        ys: paddle.Tensor,
+        next_tokens: paddle.Tensor,
+        states: List[Any],
+        xs: paddle.Tensor,
+    ) -> Tuple[paddle.Tensor, Any]:
+        """Score new token (required).
+
+        Args:
+            ys (paddle.Tensor): paddle.int64 prefix tokens (n_batch, ylen).
+            next_tokens (paddle.Tensor): paddle.int64 tokens to score (n_batch, n_token).
+            states (List[Any]): Scorer states for prefix tokens.
+            xs (paddle.Tensor):
+                The encoder feature that generates ys (n_batch, xlen, n_feat).
+
+        Returns:
+            tuple[paddle.Tensor, Any]:
+                Tuple of a score tensor for ys that has a shape `(n_batch, n_vocab)`
+                and next states for ys
+        """
+        raise NotImplementedError