feat(dup): write pegasus value in new data version for duplication (#459

)

src/base/pegasus_value_schema.h

@@ -19,16 +19,23 @@
 
 namespace pegasus {
 
-#define PEGASUS_DATA_VERSION_MAX 0u
+constexpr int PEGASUS_DATA_VERSION_MAX = 1u;
 
 /// Generates timetag in host endian.
-inline uint64_t generate_timetag(uint64_t timestamp, uint8_t cluster_id, bool delete_tag)
+/// \see comment on pegasus_value_generator::generate_value_v1
+inline uint64_t generate_timetag(uint64_t timestamp, uint8_t cluster_id, bool deleted_tag)
 {
-    return timestamp << 8u | cluster_id << 1u | delete_tag;
+    return timestamp << 8u | cluster_id << 1u | deleted_tag;
+}
+
+inline uint64_t extract_timestamp_from_timetag(uint64_t timetag)
+{
+    // 56bit: 0xFFFFFFFFFFFFFFL
+    return static_cast<uint64_t>((timetag >> 8u) & 0xFFFFFFFFFFFFFFLu);
 }
 
 /// Extracts expire_ts from rocksdb value with given version.
-/// The value schema must be in v0.
+/// The value schema must be in v0 or v1.
 /// \return expire_ts in host endian
 inline uint32_t pegasus_extract_expire_ts(uint32_t version, dsn::string_view value)
 {
@@ -55,18 +62,32 @@ pegasus_extract_user_data(uint32_t version, std::string &&raw_value, ::dsn::blob
     auto *s = new std::string(std::move(raw_value));
     dsn::data_input input(*s);
     input.skip(sizeof(uint32_t));
+    if (version == 1) {
+        input.skip(sizeof(uint64_t));
+    }
     dsn::string_view view = input.read_str();
 
     // tricky code to avoid memory copy
     std::shared_ptr<char> buf(const_cast<char *>(view.data()), [s](char *) { delete s; });
     user_data.assign(std::move(buf), 0, static_cast<unsigned int>(view.length()));
 }
 
+/// Extracts timetag from a v1 value.
+inline uint64_t pegasus_extract_timetag(int version, dsn::string_view value)
+{
+    dassert(version == 1, "data version(%d) must be v1", version);
+
+    dsn::data_input input(value);
+    input.skip(sizeof(uint32_t));
+
+    return input.read_u64();
+}
+
 /// Update expire_ts in rocksdb value with given version.
-/// The value schema must be in v0.
+/// The value schema must be in v0 or v1.
 inline void pegasus_update_expire_ts(uint32_t version, std::string &value, uint32_t new_expire_ts)
 {
-    if (version == 0) {
+    if (version == 0 || version == 1) {
         dassert_f(value.length() >= sizeof(uint32_t), "value must include 'expire_ts' header");
 
         new_expire_ts = dsn::endian::hton(new_expire_ts);
@@ -102,12 +123,16 @@ class pegasus_value_generator
 {
 public:
     /// A higher level utility for generating value with given version.
-    /// The value schema must be in v0.
-    rocksdb::SliceParts
-    generate_value(uint32_t value_schema_version, dsn::string_view user_data, uint32_t expire_ts)
+    /// The value schema must be in v0 or v1.
+    rocksdb::SliceParts generate_value(uint32_t value_schema_version,
+                                       dsn::string_view user_data,
+                                       uint32_t expire_ts,
+                                       uint64_t timetag)
     {
         if (value_schema_version == 0) {
             return generate_value_v0(expire_ts, user_data);
+        } else if (value_schema_version == 1) {
+            return generate_value_v1(expire_ts, timetag, user_data);
         } else {
             dfatal_f("unsupported value schema version: {}", value_schema_version);
             __builtin_unreachable();
@@ -136,6 +161,59 @@ class pegasus_value_generator
         return {&_write_slices[0], static_cast<int>(_write_slices.size())};
     }
 
+    /// The value schema here is designed to resolve write conflicts during duplication,
+    /// specifically, when two clusters configured as "master-master" are concurrently
+    /// writing at the same key.
+    ///
+    /// Though writings on the same key from two different clusters are rare in
+    /// real-world cases, it still gives a bad user experience when it happens.
+    /// A simple solution is to separate the writes into two halves, each cluster
+    /// is responsible for one half only. How the writes are separated is left to
+    /// users. This is simple, but unfriendly to use.
+    ///
+    /// In our design, each value is provided with a timestamp [0, 2^56-1], which
+    /// represents the data version. A write duplicated from remote cluster firstly
+    /// compares its timestamp with the current one if exists. The one with
+    /// larger timestamp wins.
+    ///
+    /// An edge case occurs when the two timestamps are completely equal, the final
+    /// result is undefined. To solve this we make 7 bits of space for cluster_id
+    /// (the globally unique id of a cluster). In case when the timestamps are equal,
+    /// the conflicts can be resolved by comparing the cluster id.
+    ///
+    /// Consider another edge case in which a record is deleted from pegasus, however
+    /// in the remote cluster this record is written with a new value:
+    ///
+    ///   A: --------- update(ts:700)---- delete ---- update duplicated from B(ts:500) --
+    ///   B: ---- update(ts:500) --------------------------------------------------------
+    ///
+    /// Since the record is removed, the stale update will successfully though
+    /// incorrectly apply. To solve this problem there's 1 bit flag marking whether the
+    /// record is deleted.
+    ///
+    /// rocksdb value (ver 1)
+    ///  = [expire_ts(uint32_t)] [timetag(uint64_t)] [user_data(bytes)]
+    ///  = [expire_ts(unit32_t)]
+    ///    [timestamp in μs (56 bit)] [cluster_id (7 bit)] [deleted_tag (1 bit)]
+    ///    [user_data(bytes)]
+    ///
+    /// \internal
+    rocksdb::SliceParts
+    generate_value_v1(uint32_t expire_ts, uint64_t timetag, dsn::string_view user_data)
+    {
+        _write_buf.resize(sizeof(uint32_t) + sizeof(uint64_t));
+        _write_slices.clear();
+
+        dsn::data_output(_write_buf).write_u32(expire_ts).write_u64(timetag);
+        _write_slices.emplace_back(_write_buf.data(), _write_buf.size());
+
+        if (user_data.length() > 0) {
+            _write_slices.emplace_back(user_data.data(), user_data.length());
+        }
+
+        return {&_write_slices[0], static_cast<int>(_write_slices.size())};
+    }
+
 private:
     std::string _write_buf;
     std::vector<rocksdb::Slice> _write_slices;

src/server/pegasus_server_impl.cpp

@@ -53,7 +53,7 @@ pegasus_server_impl::pegasus_server_impl(dsn::replication::replica *r)
     : dsn::apps::rrdb_service(r),
       _db(nullptr),
       _is_open(false),
-      _pegasus_data_version(0),
+      _pegasus_data_version(PEGASUS_DATA_VERSION_MAX),
       _last_durable_decree(0),
       _is_checkpointing(false),
       _manual_compact_svc(this)
@@ -91,6 +91,7 @@ pegasus_server_impl::pegasus_server_impl(dsn::replication::replica *r)
 
     // init rocksdb::DBOptions
     _db_opts.pegasus_data = true;
+    _db_opts.pegasus_data_version = _pegasus_data_version;
     _db_opts.create_if_missing = true;
     _db_opts.error_if_exists = false;
     _db_opts.use_direct_reads = dsn_config_get_value_bool(

src/server/pegasus_server_impl.h

@@ -13,6 +13,7 @@
 #include <dsn/dist/replication/replication.codes.h>
 #include <rrdb/rrdb_types.h>
 #include <rrdb/rrdb.server.h>
+#include <gtest/gtest_prod.h>
 
 #include "key_ttl_compaction_filter.h"
 #include "pegasus_scan_context.h"
@@ -156,6 +157,7 @@ class pegasus_server_impl : public ::dsn::apps::rrdb_service
     friend class manual_compact_service_test;
     friend class pegasus_compression_options_test;
     friend class pegasus_server_impl_test;
+    FRIEND_TEST(pegasus_server_impl_test, default_data_version);
 
     friend class pegasus_manual_compact_service;
     friend class pegasus_write_service;

src/server/pegasus_server_write.h

@@ -58,6 +58,7 @@ class pegasus_server_write : public dsn::replication::replica_base
 private:
     friend class pegasus_server_write_test;
     friend class pegasus_write_service_test;
+    friend class pegasus_write_service_impl_test;
 
     std::unique_ptr<pegasus_write_service> _write_svc;
     std::vector<put_rpc> _put_rpc_batch;

src/server/pegasus_write_service.h

@@ -38,11 +38,20 @@ struct db_write_context
 
     // Whether to compare the timetag of old value with the new write's.
     // - If true, it requires a read to the DB before write. If the old record has a larger timetag
-    // than the `remote_timetag`, the write will be ignored, otherwise it will be applied using
-    // the new timetag generated by local cluster.
+    // than `remote_timetag`, the write will be ignored, otherwise it will be applied.
     // - If false, no overhead for the write but the eventual consistency on duplication
     // is not guaranteed.
-    bool verfiy_timetag{false};
+    //
+    // This is for duplicated write only. Because under casual consistency, the former
+    // duplicated write must **happen before** the latest local write, regardless whether
+    // its timestamp is larger. This relationship can be easily proved:
+    // ```
+    //      T1(put "a") > T2(duplicated put "b") > T3(put "b" in remote cluster)
+    // ```
+    // However write conflict may still result in inconsistent data in different clusters,
+    // though those "versions" can all be considered as latest. User who requires consistency
+    // can read from one main cluster instead of reading from multiple.
+    bool verify_timetag{false};
 
     static inline db_write_context empty(int64_t d) { return create(d, 0); }
 
@@ -62,7 +71,7 @@ struct db_write_context
         db_write_context ctx;
         ctx.decree = decree;
         ctx.remote_timetag = remote_timetag;
-        ctx.verfiy_timetag = verify_timetag;
+        ctx.verify_timetag = verify_timetag;
         return ctx;
     }
 
@@ -148,6 +157,7 @@ class pegasus_write_service
 
 private:
     friend class pegasus_write_service_test;
+    friend class pegasus_write_service_impl_test;
     friend class pegasus_server_write_test;
 
     pegasus_server_impl *_server;

src/server/pegasus_write_service_impl.h

@@ -12,6 +12,7 @@
 
 #include <dsn/utility/fail_point.h>
 #include <dsn/utility/string_conv.h>
+#include <gtest/gtest_prod.h>
 
 namespace pegasus {
 namespace server {
@@ -20,6 +21,32 @@ namespace server {
 static constexpr int FAIL_DB_WRITE_BATCH_PUT = -101;
 static constexpr int FAIL_DB_WRITE_BATCH_DELETE = -102;
 static constexpr int FAIL_DB_WRITE = -103;
+static constexpr int FAIL_DB_GET = -104;
+
+struct db_get_context
+{
+    // value read from DB.
+    std::string raw_value;
+
+    // is the record found in DB.
+    bool found{false};
+
+    // the expiration time encoded in raw_value.
+    uint32_t expire_ts{0};
+
+    // is the record expired.
+    bool expired{false};
+};
+
+inline int get_cluster_id_if_exists()
+{
+    // cluster_id is 0 if not configured, which means it will accept writes
+    // from any cluster as long as the timestamp is larger.
+    static auto cluster_id_res =
+        dsn::replication::get_duplication_cluster_id(dsn::replication::get_current_cluster_name());
+    static uint64_t cluster_id = cluster_id_res.is_ok() ? cluster_id_res.get_value() : 0;
+    return cluster_id;
+}
 
 class pegasus_write_service::impl : public dsn::replication::replica_base
 {
@@ -513,14 +540,37 @@ class pegasus_write_service::impl : public dsn::replication::replica_base
         FAIL_POINT_INJECT_F("db_write_batch_put",
                             [](dsn::string_view) -> int { return FAIL_DB_WRITE_BATCH_PUT; });
 
-        if (ctx.verfiy_timetag) {
-            // TBD(wutao1)
+        uint64_t new_timetag = ctx.remote_timetag;
+        if (!ctx.is_duplicated_write()) { // local write
+            new_timetag = generate_timetag(ctx.timestamp, get_cluster_id_if_exists(), false);
+        }
+
+        if (ctx.verify_timetag &&         // needs read-before-write
+            _pegasus_data_version >= 1 && // data version 0 doesn't support timetag.
+            !raw_key.empty()) {           // not an empty write
+
+            db_get_context get_ctx;
+            int err = db_get(raw_key, &get_ctx);
+            if (dsn_unlikely(err != 0)) {
+                return err;
+            }
+            // if record exists and is not expired.
+            if (get_ctx.found && !get_ctx.expired) {
+                uint64_t local_timetag =
+                    pegasus_extract_timetag(_pegasus_data_version, get_ctx.raw_value);
+
+                if (local_timetag >= new_timetag) {
+                    // ignore this stale update with lower timetag,
+                    // and write an empty record instead
+                    raw_key = value = dsn::string_view();
+                }
+            }
         }
 
         rocksdb::Slice skey = utils::to_rocksdb_slice(raw_key);
         rocksdb::SliceParts skey_parts(&skey, 1);
-        rocksdb::SliceParts svalue =
-            _value_generator.generate_value(_pegasus_data_version, value, db_expire_ts(expire_sec));
+        rocksdb::SliceParts svalue = _value_generator.generate_value(
+            _pegasus_data_version, value, db_expire_ts(expire_sec), new_timetag);
         rocksdb::Status s = _batch.Put(skey_parts, svalue);
         if (dsn_unlikely(!s.ok())) {
             ::dsn::blob hash_key, sort_key;
@@ -570,6 +620,37 @@ class pegasus_write_service::impl : public dsn::replication::replica_base
         return status.code();
     }
 
+    // The resulted `expire_ts` is -1 if record is expired.
+    int db_get(dsn::string_view raw_key,
+               /*out*/ db_get_context *ctx)
+    {
+        FAIL_POINT_INJECT_F("db_get", [](dsn::string_view) -> int { return FAIL_DB_GET; });
+
+        rocksdb::Status s = _db->Get(_rd_opts, utils::to_rocksdb_slice(raw_key), &(ctx->raw_value));
+        if (dsn_likely(s.ok())) {
+            // success
+            ctx->found = true;
+            ctx->expire_ts = pegasus_extract_expire_ts(_pegasus_data_version, ctx->raw_value);
+            if (check_if_ts_expired(utils::epoch_now(), ctx->expire_ts)) {
+                ctx->expired = true;
+            }
+            return 0;
+        }
+        if (s.IsNotFound()) {
+            // NotFound is an acceptable error
+            ctx->found = false;
+            return 0;
+        }
+        ::dsn::blob hash_key, sort_key;
+        pegasus_restore_key(::dsn::blob(raw_key.data(), 0, raw_key.size()), hash_key, sort_key);
+        derror_rocksdb("Get",
+                       s.ToString(),
+                       "hash_key: {}, sort_key: {}",
+                       utils::c_escape_string(hash_key),
+                       utils::c_escape_string(sort_key));
+        return s.code();
+    }
+
     void clear_up_batch_states(int64_t decree, int err)
     {
         if (!_update_responses.empty()) {
@@ -715,6 +796,9 @@ class pegasus_write_service::impl : public dsn::replication::replica_base
 private:
     friend class pegasus_write_service_test;
     friend class pegasus_server_write_test;
+    friend class pegasus_write_service_impl_test;
+    FRIEND_TEST(pegasus_write_service_impl_test, put_verify_timetag);
+    FRIEND_TEST(pegasus_write_service_impl_test, verify_timetag_compatible_with_version_0);
 
     const std::string _primary_address;
     const uint32_t _pegasus_data_version;
@@ -725,7 +809,6 @@ class pegasus_write_service::impl : public dsn::replication::replica_base
     rocksdb::ReadOptions &_rd_opts;
     volatile uint32_t _default_ttl;
     ::dsn::perf_counter_wrapper &_pfc_recent_expire_count;
-
     pegasus_value_generator _value_generator;
 
     // for setting update_response.error after committed.

src/server/test/pegasus_server_impl_test.cpp

@@ -59,5 +59,10 @@ class pegasus_server_impl_test : public pegasus_server_test_base
 
 TEST_F(pegasus_server_impl_test, test_table_level_slow_query) { test_table_level_slow_query(); }
 
+TEST_F(pegasus_server_impl_test, default_data_version)
+{
+    ASSERT_EQ(_server->_pegasus_data_version, 1);
+}
+
 } // namespace server
 } // namespace pegasus