diff --git a/tensorboard/uploader/proto/BUILD b/tensorboard/uploader/proto/BUILD
index f96ce1264a..cdc7212e1a 100644
--- a/tensorboard/uploader/proto/BUILD
+++ b/tensorboard/uploader/proto/BUILD
@@ -10,6 +10,7 @@ exports_files(["LICENSE"])
 tb_proto_library(
     name = "protos_all",
     srcs = [
+        "blob.proto",
         "export_service.proto",
         "scalar.proto",
         "server_info.proto",
diff --git a/tensorboard/uploader/proto/blob.proto b/tensorboard/uploader/proto/blob.proto
new file mode 100644
index 0000000000..f218306f74
--- /dev/null
+++ b/tensorboard/uploader/proto/blob.proto
@@ -0,0 +1,14 @@
+syntax = "proto3";
+
+package tensorboard.service;
+
+enum BlobState {
+  // Object state is unknown. This value should never be used; it is present
+  // only as a proto3 best practice.
+  // See https://developers.google.com/protocol-buffers/docs/proto3#enum
+  UNKNOWN = 0;
+  // Object is being written and not yet finalized.
+  UNFINALIZED = 1;
+  // Object is finalized.
+  CURRENT = 2;
+}
\ No newline at end of file
diff --git a/tensorboard/uploader/proto/write_service.proto b/tensorboard/uploader/proto/write_service.proto
index b5dd3b0bce..6f168e62f6 100644
--- a/tensorboard/uploader/proto/write_service.proto
+++ b/tensorboard/uploader/proto/write_service.proto
@@ -8,6 +8,7 @@ package tensorboard.service;
 import "tensorboard/uploader/proto/export_service.proto";
 import "tensorboard/uploader/proto/scalar.proto";
 import "tensorboard/uploader/proto/tensor.proto";
+import "tensorboard/uploader/proto/blob.proto";
 import "tensorboard/compat/proto/summary.proto";
 
 // Service for writing data to TensorBoard.dev.
@@ -30,6 +31,16 @@ service TensorBoardWriterService {
   rpc WriteScalar(WriteScalarRequest) returns (WriteScalarResponse) {}
   // Request additional tensor data be stored in TensorBoard.dev.
   rpc WriteTensor(WriteTensorRequest) returns (WriteTensorResponse) {}
+  // Request to obtain a specific BlobSequence entry, creating it if needed,
+  // to be subsequently populated with blobs.
+  rpc GetOrCreateBlobSequence(GetOrCreateBlobSequenceRequest)
+      returns (GetOrCreateBlobSequenceResponse) {}
+  // Request the current status of blob data being stored in TensorBoard.dev,
+  // to support resumable uploads.
+  rpc GetBlobMetadata(GetBlobMetadataRequest)
+      returns (GetBlobMetadataResponse) {}
+  // Request additional blob data be stored in TensorBoard.dev.
+  rpc WriteBlob(stream WriteBlobRequest) returns (stream WriteBlobResponse) {}
   // Request that the calling user and all their data be permanently deleted.
   // Used for testing purposes.
   rpc DeleteOwnUser(DeleteOwnUserRequest) returns (DeleteOwnUserResponse) {}
@@ -190,6 +201,148 @@ message WriteTensorResponse {
   // This is empty on purpose.
 }
 
+// Writing a blob sequence
+// =======================
+//  * The caller requests creation of a new BlobSequence, via the
+//    GetOrCreateBlobSequence RPC. The response provides a new BlobSequence ID.
+//  * The caller writes each blob in the sequence separately via the WriteBlob
+//    RPC as described below, providing the blob sequence ID and sequence index
+//    for each blob.
+//  * Multiple WriteBlob streams may operate in parallel (for different blobs),
+//    with no constraint on their ordering.
+
+// Resuming a partially written blob sequence
+// ==========================================
+//  * The client starts the upload process with a new GetOrCreateBlobSequence
+//    RPC.
+//  * The blob sequences are uniquely keyed by experiment, run, tag, and step.
+//    Since this is a second (or later) attempt regarding an existing sequence,
+//    a new sequence is not created.  Instead the existing sequence ID is
+//    returned.
+//  * The client attempts to upload each blob in the sequence.  These uploads
+//    resume or may be skipped entirely, as needed, according to the below
+//    procedure.
+
+// Writing a blob
+// ==============
+//  * The caller instantiates a WriteBlob RPC, which is a bidirectional stream.
+//  * The caller sends one or more WriteBlobRequests, providing the blob data
+//    in an ordered sequence of chunks.
+//  * The caller receives a sequence of WriteBlobResponses in return. Each
+//    WriteBlobResponse corresponds to one WriteBlobRequest in order, reporting
+//    the state of the write operation.
+//  * The caller should not wait to verify that each response reports the
+//    expected status and size before sending the next request. Doing so reduces
+//    throughput, with little benefit since any mismatch on the server side will
+//    cause the RPC to throw an error anyway.
+//  * Nonetheless the caller may wish to validate these values, if provided,
+//    whenever the response does arrive.
+//  * The request size should be less than 4 MiB (2^20 = 4,194,304), because
+//    that is the default request limit for gRPC. Accounting for the other
+//    fields of the request, then, and leaving some padding for safety, the data
+//    chunk should be somewhat smaller-- perhaps simply 4e6 bytes.
+//  * The last of these requests must set finalize_object=True, and the prior
+//    ones must not.
+//  * For a blob <= 4MB, a single request suffices, so it is the last chunk.
+
+// Resuming a partial blob upload
+// ==============================
+//  * When the client begins an upload by the usual method above, it may turn
+//    out that the data chunk is redundant with a previous partial upload.
+//    The serve will raise an error in this case, closing the stream.
+//  * At this point the client should issue a GetBlobMetadataRequest.  The
+//    GetBlobMetadataResponse reports the state, size, and crc32c of the object
+//    so far, i.e. taking into account any previously uploaded data.
+//  * The client checks whether any data remains to upload, and optionally
+//    validates the cumulative crc32c against the local file (up to the given
+//    offset).
+//  * If needed, the client starts a new WriteBlob stream, starting from the
+//    specified offset.
+//  * The client may choose to call GetBlobMetadata before starting any
+//    WriteBlob stream, to avoid the potential for wasted transfer of a
+//    redundant chunk, at the cost of the initial roundtrip to obtain the
+//    status.  It is up to the client to weigh this tradeoff.
+
+// Obtain a unique ID for a blob sequence, given the composite key
+// (experiment_id, run, tag, step).  If such a blob sequence already exists,
+// return its ID.  If not, create it first, and return the new ID.
+message GetOrCreateBlobSequenceRequest {
+  // Service-wide unique identifier of an uploaded log dir.
+  // eg: "1r9d0kQkh2laODSZcQXWP"
+  string experiment_id = 1;
+  // The name of the run to which the blob sequence belongs, for example
+  // "/some/path/mnist_experiments/run1/".
+  string run = 2;
+  // The name of the tag to which the blob sequence belongs, for example "loss".
+  string tag = 3;
+  // Step index within the run.
+  int64 step = 4;
+  // The total number of elements expected in the sequence.
+  // This effectively delares a number of initially empty 'upload slots',
+  // to be filled with subsequent WriteBlob RPCs.
+  int64 final_sequence_length = 5;
+  // Note that metadata.plugin_data.content does not carry the payload.
+  .tensorboard.SummaryMetadata metadata = 6;
+}
+
+message GetOrCreateBlobSequenceResponse {
+  // A unique ID for the the requested blob sequence.
+  string blob_sequence_id = 1;
+}
+
+message GetBlobMetadataRequest {
+  // The ID of the BlobSequence of which this blob is a member.
+  string blob_sequence_id = 1;
+  // The position of this Blob within the BlobSequence
+  int64 index = 2;
+}
+
+message GetBlobMetadataResponse {
+  // State of the object (still appending vs. complete).
+  BlobState blob_state = 1;
+  // Size of the object in bytes.  In the case of a partial upload, this
+  // reflects only the data actually received so far.
+  int64 size = 2;
+  // crc32c of the blob data stored so far, i.e. over the byte range [0, size).
+  fixed32 crc32c = 3;
+}
+
+// A single chunk of the blob write stream.
+//
+// Note that the WriteBlobRequest does not mirror the nested structure of
+// WriteScalarRequest, because we only ever send one blob at a time.
+message WriteBlobRequest {
+  // The ID of the BlobSequence of which this blob is a member.
+  string blob_sequence_id = 1;
+  // The position of this Blob within the BlobSequence
+  int64 index = 2;
+  // The bytes in this chunk.
+  bytes data = 3;
+  // The position in the blob where this chunk begins.
+  // This must equal the sum of the sizes of the chunks sent so far.  Ignored
+  // if no data is provided.
+  int64 offset = 4;
+  // CRC32C of current data buffer. Clients must include the crc32c for every
+  // data buffer, to protect against data corruption.  Note that for multi-shot
+  // writes, specifying the crc32c for every data buffer provides stronger
+  // protection than just providing the final_crc32c at the end of the upload.
+  fixed32 crc32c = 5;
+  // Indicates that this is the last chunk of the stream.
+  bool finalize_object = 6;
+  // CRC32C of the entire blob.  Required, to protect against data corruption.
+  // This should be set only when finalize_object=True.
+  fixed32 final_crc32c = 7;
+}
+
+message WriteBlobResponse {
+  // State of the object (still appending vs. complete).
+  BlobState blob_state = 1;
+  // Size of the object in bytes.  This is the sum of the chunk sizes
+  // received from the stream so far.  In the response to the final chunk,
+  // this size should equal the total size of the blob.
+  int64 size = 2;
+}
+
 // Requests that the calling user and all their data be permanently deleted.
 message DeleteOwnUserRequest {
   // This is empty on purpose.