1 // Copyright (c) 2012 The Chromium Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 // 5 // Protobuf representation of the UniquePosition class. 6 7 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change 8 // any fields in this file. 9 10 syntax = "proto2"; 11 12 option optimize_for = LITE_RUNTIME; 13 option retain_unknown_fields = true; 14 15 package sync_pb; 16 17 // A UniquePosition is a string of bytes. 18 // 19 // Unique positions are unique per-item, since they are guaranteed to end with a 20 // fixed-length suffix that is unique per-item. The position string may not end 21 // with a '\0' byte. 22 // 23 // Prior to the suffix is a series of arbitrary bytes of arbitrary length. 24 // Items under the same parent are positioned relative to each other by a 25 // lexicographic comparison of their UniquePosition values. 26 message UniquePosition { 27 // History: 28 // 29 // Unique positions were first introduced in M28. This change was rolled out 30 // in such a way that it would try to maintain backwards compatibilty with 31 // clients that understood only the old int64-based positions. 32 // 33 // At first, clients supported only the 'value' field. This version never 34 // made it to stable. We later added support for the 'compressed_value' 35 // field, and clients would populate either one or the other. 36 // 37 // In M30, we added the custom_compressed_v1 representation. This 38 // representation was better than the previous implementations in almost every 39 // way. However, we could not use it right away, since older clients would 40 // not understand it. We decided to write both the old-style ('value' or 41 // 'custom_compressed') representation and the 'custom_compressed_v1' 42 // repersentations to every protobuf during the transition period. Protobufs 43 // written during this transition period would be readable by clients who 44 // understand at least one of the two formats. 45 // 46 // In M33, we dropped support for writing the backwards-compatibility fields. 47 // Protobufs written by this version or later are not be intelligible by 48 // clients with version M29 or older. Those clients will end up making use of 49 // the old int64 position fallback mechanism. 50 51 // The uncompressed string of bytes representing the position. 52 // 53 // Deprecated. See history note above. 54 optional bytes value = 1; 55 56 // The client may choose to write a compressed position to this field instead 57 // of populating the 'value' above. If it chooses to use compression, the 58 // 'value' field above must be empty. The position value will be compressed 59 // with gzip and stored in the compressed_value field. The position's 60 // uncompressed length must be specified and written to the 61 // uncompressed_length field. 62 // 63 // Deprecated. See history note above. 64 optional bytes compressed_value = 2; 65 optional uint64 uncompressed_length = 3; 66 67 // This encoding uses compression scheme designed especially for unique 68 // positions. It has the property that X < Y precisely when Compressed(X) < 69 // Compressed(Y), which is very useful when the most common operation is to 70 // compare these positions against each other. Their values may remain 71 // compressed in memory. 72 // 73 // The compression scheme is implemented and documented in 74 // sync/internal_api/base/unique_position.cc. 75 // 76 // As of M30, this is the preferred encoding. Newer clients may continue to 77 // populate the 'value' and 'compressed_value' fields to ensure backwards 78 // compatibility, but they will always try to read from this field first. 79 optional bytes custom_compressed_v1 = 4; 80 } 81
