feat(ds): implement raft-based replication #12361

keynslug · 2024-01-21T23:07:33Z

Summary

Uses Raft through https://github.com/rabbitmq/ra to replicate writes across nodes: each shard is now a Raft consensus group. Replicas are guaranteed to not diverge, therefore we're free to pick any replica to perform reads.

Total order of messages in each shard
Session replay tests work
All tests are green

Roadmap

This is mostly out-of-scope for this PR (maybe except for first two), but good to have written down.

Measure performance impact.

Read performance shouldn't really suffer. Things to explore: issue release_cursor less often, make smarter assumptions about shard servers, do not waste cycles on constructing names / IDs.
Perform rest of shard-level operations through consensus.

Namely: adding generations. At first look it seems important because of snapshotting. Initially we could make it work synchronously, where adding a generation will block message writes. Then think about doing asynchronously while preserving safety.
Solve snapshotting story.

We could relax strictness and take the most recent snapshots, instead of taking them at the specified Raft index. Supposedly, this should be safe as long as writes are idempotent, and repeating write operations already present in a snapshot will not accidentally cause write duplication. At first glance, the only thing needed is making add generation part of Raft logs.
Design good first start UX story.

In other words: how operators are supposed to ensure that the cluster they want to deploy will start and fairly allocate shards between them. Currently node in the cluster does not know how large the cluster is supposed to be, and could steal too much shards / replicas.
Perform cluster-wide changes through consensus.

Related to the above. Super important to unlock safe rebalancing. Quite tricky because of uncertainty about initial cluster membership.
Implement rebalancing.
Skip RocksDB WAL.

This is a rather long-term goal. We already have a Raft log which serves basically the same purpose, so disabling RocksDB WAL and taking control over dumping memtables / releasing Raft log entries should help to avoid keeping two logs.

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

Added tests for the changes
~~Added property-based tests for code which performs user input validation~~
Changed lines covered in coverage report
Change log has been added to changes/(ce|ee)/(feat|perf|fix|breaking)-<PR-id>.en.md files
For internal contributor: there is a jira ticket to track this change
~~Created PR to emqx-docs if documentation update is required, or link to a follow-up jira ticket~~
Schema changes are backward compatible

thalesmg · 2024-01-25T19:13:58Z

apps/emqx_durable_storage/src/emqx_ds_storage_bitfield_lts.erl

@@ -412,11 +410,10 @@ make_key(#s{keymappers = KeyMappers, trie = Trie}, #message{timestamp = Timestam
 ]) ->
    binary().
 make_key(KeyMapper, TopicIndex, Timestamp, Varying) ->
-    UniqueInteger = erlang:unique_integer([monotonic, positive]),


Q: it's still possible to have duplicate timestamps at high concurrency/rates, so shouldn't we keep this random part?

Since we want total order of messages per stream, this PR just enforces total order per shard through allocating unique pseudo-realtime timestamps to messages (admittedly, in a quite ugly way). Since we don't want timestamps to occasionally jump backwards during leader election it's important to have that as part of Raft log / state, and uniqueness (i.e. strict monotonicity) is trivial to add on top of that.

With that we don't need this extra uniqueness, which furthermore is not really guaranteed to be unique across shard's replica set.

I'm thinking about if that could also be a problem, now from the opposite side. 😅

When moving session data from mnesia to DS, I took advantage of the fact that the message timestamps are respected by the storage layer to upsert keys. Now that emqx_ds_replication_layer:assign_timestamps enforces unique timestamps, it'll no longer be possible to upsert a key if one wants to do that in a particular DB. 🤔

Now that emqx_ds_replication_layer:assign_timestamps enforces unique timestamps, it'll no longer be possible

Conceptually, this needs to be enforced only for messages that needs to be replayed in a strict order. If we need to accommodate for another storage semantics, nothing should stop us from expanding API here, e.g. adding flags or properties to store batch command.

thalesmg · 2024-01-25T19:29:38Z

apps/emqx_durable_storage/src/emqx_ds_replication_layer_meta.erl

@@ -430,17 +453,23 @@ ensure_site() ->
 -spec create_shards(emqx_ds:db(), pos_integer(), pos_integer()) -> ok.
 create_shards(DB, NShards, ReplicationFactor) ->
    Shards = [integer_to_binary(I) || I <- lists:seq(0, NShards - 1)],
-    AllSites = sites(),
+    AllSites = mnesia:match_object(?NODE_TAB, #?NODE_TAB{_ = '_'}, read),
+    Nodes = mria_mnesia:running_nodes(),


Suggested change

Nodes = mria_mnesia:running_nodes(),

Nodes = emqx:running_nodes(),

?

thalesmg · 2024-01-25T19:57:49Z

apps/emqx_durable_storage/src/emqx_ds_replication_layer.erl

+        log_init_args => #{uid => UID}
+    }),
+    case Servers of
+        [LocalServer | _] ->


Q: why do we trigger election if Servers/replica set has the local server at the head? Is it a way to make only one server trigger the election?

Yes. Apparently ra servers do not start elections automatically, and we need to poke them, thus this quick attempt to trigger it only once. Needs some testing to verify that it works well enough.

thalesmg · 2024-01-25T20:29:36Z

apps/emqx_durable_storage/src/emqx_ds_replication_layer.erl

+%%
+
+init(#{db := DB, shard := Shard}) ->
+    _ = erlang:put(emqx_ds_db_shard, {DB, Shard}),


Q: is there a particular reason not to put this in the state map?

My line of thinking is that State is essentially a product of applying Raft log entries, and is periodically snapshotted and dumped to disk. This tuple feels odd as part of state / snapshot, it's more like a runtime-only information needed to locate relevant Erlang processes. On the other hand I agree that it feels hacky to put it in the process dictionary.

I wouldn't object to using process dictionary too much, as long as the terms are static.
Process dictionary becomes rather sticky when it's used to store temporary data, because it doesn't get cleaned up on exception. This is not the case here.

savonarola · 2024-01-30T11:01:14Z

apps/emqx_durable_storage/src/emqx_ds_replication_layer.erl

+    Messages = assign_timestamps(Timestamp, MessagesIn),
+    Result = emqx_ds_storage_layer:store_batch(erlang:get(emqx_ds_db_shard), Messages, Options),
+    %% NOTE: Last assigned timestamp.
+    NLatest = Timestamp + length(Messages) - 1,


Nit: m.b. return from NLatest from assign_timestamps

ieQu1 · 2024-01-31T14:18:25Z

apps/emqx_durable_storage/src/emqx_ds.erl

@@ -94,9 +94,11 @@
 -type next_result() :: next_result(iterator()).

 %% Timestamp
+%% Each message must have unique timestamp.


Storage layer introduces a "unique integer" of its own, and keeps the original timestamp. (Admittedly, it was a temporary kludge) It looks like this PR shifts the responsibility of making the timestamps unique to the application.

Some thoughts on this:

I think emqx_ds should consume and return unchanged messages, including the timestamp

We shouldn't move the responsibility of making message timestamps unique to the API consumer (it will introduce a footgun for the API users); it's better if the replication layer takes care of it in a discreet manner.

Moreover, if the replication layer is able to create its own "internal" timestamps, we have more options in the way of dealing with the clock skews. For example, (in the future) we can order the messages according to the leader's clock, which will make things more predictable.

this PR shifts the responsibility of making the timestamps unique to the application

To the replication layer actually.

We shouldn't move the responsibility of making message timestamps unique to the API consumer

Agree. This PR doesn't move the responsibility though. The replication layer modifies message timestamps to ensure uniqueness. API consumer will definitely be surprised though, but it's a PoC-grade hack at this point.

we can order the messages according to the leader's clock

They are already ordered according to the leader's clock (to be more precise, shard's consensus group tracks strictly monotonically increasing clock).

thalesmg · 2024-03-11T20:23:56Z

apps/emqx_utils/src/emqx_utils_stream.erl

@@ -91,6 +93,31 @@ map(F, S) ->
        end
    end.

+%% @doc Zip a list of streams into a stream producing lists of their respective values.
+%% The resulting stream is as long as the shortest of the input streams.
+-spec zip([stream(X)]) -> stream([X]).


Nit: with this name, I'd expect the typespec to be something like zip([stream(_)]) -> stream(tuple()), similar to lists:zip, Enum.zip and others. But I see how this has the same "shape" as a stream of tuples. 🤔

Also, this function seems to be unused in this PR?

I should probably rename it. Agree that it brokes expectations of what _zip_s are, however I found that having it emit rows as lists is a bit more usable overall, especially given it's generic in the number of things being zipped (e.g. one can do zipwith/2 through erlang:apply/2).

Also, this function seems to be unused in this PR?

Yeah it was used in the previous iteration. Do you think it's better to drop it?

Do you think it's better to drop it?

I think it's fine to leave it here, since it's tested.

apps/emqx_durable_storage/src/emqx_ds_replication_layer_meta.erl

ieQu1 · 2024-03-18T15:00:11Z

apps/emqx_durable_storage/src/emqx_ds_replication_layer_meta.erl

@@ -431,8 +380,8 @@ ensure_site() ->
        {ok, [Site]} ->
            ok;
        _ ->
-            Site = crypto:strong_rand_bytes(8),
-            logger:notice("Creating a new site with ID=~s", [base64:encode(Site)]),
+            Site = binary:encode_hex(crypto:strong_rand_bytes(4)),


Why change from 8 to 4? Not a huge win in terms of storage, but probability of collision in a 20-node cluster in terms of birthday paradox goes up from ~1e-17 to ~1e-8.

ieQu1 · 2024-03-18T15:05:44Z

apps/emqx_durable_storage/src/emqx_ds_replication_layer_shard.erl

+        undefined ->
+            ok = persistent_term:put(__X_Key, __X_Value = (EXPR)),
+            __X_Value;
+        __X_Value ->


Dangerous macro. If someone invokes it several times in the same function, they will get a badmatch. I would prefer it to be rewritten as a function that takes an argument possibly created with the help of a macro.
Also, from reading it I don't see why it should be a macro.

thalesmg · 2024-03-18T19:37:11Z

apps/emqx_durable_storage/src/emqx_ds_replication_shard_allocator.erl

+                }
+            ),
+            {noreply, State, ?ALLOCATE_RETRY_TIMEOUT}
+    end.


Suggested change

end.

end;

handle_info(_Info, State) ->

{noreply, State}

end.

thalesmg · 2024-03-18T19:46:25Z

apps/emqx_durable_storage/src/emqx_ds_replication_shard_allocator.erl

+            ok = start_egresses(DB, Shards),
+            ok = save_db_meta(DB, Shards),
+            ok = save_shards_meta(DB, Shards),
+            State#{shards => Shards, status := ready};


Nit:

Suggested change

State#{shards => Shards, status := ready};

{ok, State#{shards => Shards, status := ready}};

thalesmg · 2024-03-18T19:50:12Z

apps/emqx_durable_storage/src/emqx_ds_replication_layer.erl

-    do_delete_next_v4/5
+    do_delete_next_v4/5,
+
+    %% FIXME


Still relevant?

thalesmg · 2024-03-18T20:02:10Z

apps/emqx_durable_storage/src/emqx_ds_storage_bitfield_lts.erl

@@ -412,11 +410,10 @@ make_key(#s{keymappers = KeyMappers, trie = Trie}, #message{timestamp = Timestam
 ]) ->
    binary().
 make_key(KeyMapper, TopicIndex, Timestamp, Varying) ->
-    UniqueInteger = erlang:unique_integer([monotonic, positive]),


I'm thinking about if that could also be a problem, now from the opposite side. 😅

When moving session data from mnesia to DS, I took advantage of the fact that the message timestamps are respected by the storage layer to upsert keys. Now that emqx_ds_replication_layer:assign_timestamps enforces unique timestamps, it'll no longer be possible to upsert a key if one wants to do that in a particular DB. 🤔

thalesmg · 2024-03-18T20:06:03Z

apps/emqx_durable_storage/src/emqx_ds_builtin_db_sup.erl

    {ok, {SupFlags, Children}}.

+start_ra_system(DB, #{replication_options := ReplicationOpts}) ->
+    DataDir = filename:join([emqx:data_dir(), DB, dsrepl]),


Suggested change

DataDir = filename:join([emqx:data_dir(), DB, dsrepl]),

DataDir = filename:join([emqx_ds:base_dir(), DB, dsrepl]),

thalesmg · 2024-03-18T20:23:42Z

apps/emqx_durable_storage/src/emqx_ds_storage_layer.erl

+-spec handle_update_config(server_state(), emqx_ds:time(), emqx_ds:create_db_opts()) ->
+    server_state().
+handle_update_config(S0 = #s{schema = Schema}, Since, Options) ->
+    Prototype = maps:get(storage, Options),
+    S = S0#s{schema = Schema#{prototype := Prototype}},
+    handle_add_generation(S, Since).


Now this doesn't always return the server state: it may return {error, Reason}.

thalesmg · 2024-03-18T20:24:34Z

apps/emqx_durable_storage/src/emqx_ds_storage_layer.erl

-    Since = emqx_message:timestamp_now(),
-    S = add_generation(S0, Since),
+handle_call(#call_add_generation{since = Since}, _From, S0) ->
+    S = #s{} = handle_add_generation(S0, Since),


May need to check for {error, _} here.

thalesmg · 2024-03-18T20:24:39Z

apps/emqx_durable_storage/src/emqx_ds_storage_layer.erl

-    Since = emqx_message:timestamp_now(),
-    S = add_generation(S1, Since),
+handle_call(#call_update_config{since = Since, options = Options}, _From, S0) ->
+    S = #s{} = handle_update_config(S0, Since, Options),


May need to check for {error, _} here.

thalesmg · 2024-03-18T20:33:07Z

apps/emqx_durable_storage/src/emqx_ds_replication_layer.erl

@@ -469,10 +439,11 @@ do_delete_next_v4(DB, Shard, Iter, Selector, BatchSize) ->

 -spec do_add_generation_v2(emqx_ds:db()) -> ok | {error, _}.
 do_add_generation_v2(DB) ->
-    MyShards = emqx_ds_replication_layer_meta:my_owned_shards(DB),
+    %% FIXME


Still relevant?

Still very rough but mostly working.

This is needed to ensure total message order for a shard, and guarantee that no messages will be written "in the past". which may break replay consistency.

Before this commit the most likely shard allocation outcome was that all shard are allocated to just one node.

It doesn't feel right, but right now is the easiest way to have it in the scope of `apply/3`, because `init/1` doesn't actually invoked for ra machines recovered from the existing log / snapshot.

That starts shard and egress processes only when shards are fully allocated.

1. This avoids the need to deserialize the message to get the timestamp. 2. It also makes possible to decouple the storage key timestamp from the message timestamp, which might be useful for replication purposes.

ieQu1 · 2024-03-19T20:43:16Z

apps/emqx_durable_storage/src/emqx_ds_replication_layer_shard.erl

+start_link(DB, Shard, Opts) ->
+    gen_server:start_link(?MODULE, {DB, Shard, Opts}, []).
+
+shard_servers(DB, Shard) ->


Nit (can be done in a follow-up): it would be nice to have type specs for all the exported functions.

Storage also becomes a bit more pure, depending on the upper layer to provide the timestamps, which also makes it possible to handle more operations idempotently.

In order to minimize chances of site id collision to practically zero.

Co-Authored-By: Thales Macedo Garitezi <thalesmg@gmail.com>

Also update few outdated typespecs. Also make error reasons easier to comprehend.

Also annotate internal exports with comments according with their intended use.

Before this commit, messages in the current batch will be retried as part of next batch. This could have led to message duplication which is probably not what the user wants by default.

emqxqa · 2024-05-08T01:28:59Z

Test Execution

thalesmg reviewed Jan 25, 2024

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keynslug force-pushed the ft/EMQX-11756/emqx-ds-replication branch from c199c0b to 58bd42b Compare February 12, 2024 18:24

keynslug force-pushed the ft/EMQX-11756/emqx-ds-replication branch from 0e832db to d6327fc Compare March 8, 2024 18:09

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keynslug force-pushed the ft/EMQX-11756/emqx-ds-replication branch from d6327fc to 9242cf4 Compare March 12, 2024 17:43

keynslug marked this pull request as ready for review March 12, 2024 19:07

keynslug requested review from a team and lafirest as code owners March 12, 2024 19:07

keynslug force-pushed the ft/EMQX-11756/emqx-ds-replication branch from 9242cf4 to f9df2a7 Compare March 12, 2024 19:47

keynslug changed the title ~~wip: implement raft-based replication~~ feat(ds): implement raft-based replication Mar 18, 2024

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keynslug added 9 commits March 19, 2024 20:09

feat(dsrepl): implement raft-based replication

146f082

Still very rough but mostly working.

fix(dsrepl): reassign timestamp at the time of submission

be793e4

This is needed to ensure total message order for a shard, and guarantee that no messages will be written "in the past". which may break replay consistency.

feat(dsrepl): allocate shards once predefined number of sites online

5e94bdb

Before this commit the most likely shard allocation outcome was that all shard are allocated to just one node.

feat(dsrepl): manage generations / db config through ra machine

e6c2c2f

feat(dsrepl): cache shard metadata in persistent terms

d19128e

fix(dsrepl): make db + shard part of machine state

4dafbf2

It doesn't feel right, but right now is the easiest way to have it in the scope of `apply/3`, because `init/1` doesn't actually invoked for ra machines recovered from the existing log / snapshot.

feat(dsrepl): move shard allocation to separate process

3b59cf2

That starts shard and egress processes only when shards are fully allocated.

fix(boot): add emqx_durable_storage to the list of restarted apps

6c6ea50

fix(dsrepl): tolerate trigger election timeouts for existing servers

f89909f

feat(ds-lts): extract timestamp from storage key itself

3cb36a5

1. This avoids the need to deserialize the message to get the timestamp. 2. It also makes possible to decouple the storage key timestamp from the message timestamp, which might be useful for replication purposes.

keynslug force-pushed the ft/EMQX-11756/emqx-ds-replication branch 2 times, most recently from b3df14a to 5a46b82 Compare March 19, 2024 20:19

ieQu1 reviewed Mar 19, 2024

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ieQu1 previously approved these changes Mar 19, 2024

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keynslug dismissed ieQu1’s stale review via 082d3f3 March 20, 2024 11:32

keynslug force-pushed the ft/EMQX-11756/emqx-ds-replication branch from 5a46b82 to 082d3f3 Compare March 20, 2024 11:32

keynslug and others added 15 commits March 20, 2024 13:20

feat(dsrepl): make storage layer unaware of granularity of time

74881e8

Storage also becomes a bit more pure, depending on the upper layer to provide the timestamps, which also makes it possible to handle more operations idempotently.

feat(dsrepl): use more straightforward way to drop ra shards

611b3f0

chore(dsrepl): make dialyzer a bit happier

1b64703

fix(dsrepl): split shard allocator into a separate module

ac9700d

fix(build): sync mix dependencies

46d9adb

fix(dsrepl): increase replication site id bitsize back

0e18bd6

In order to minimize chances of site id collision to practically zero.

refactor(dsrepl): make shard allocator more robust and consistent

404e919

Co-Authored-By: Thales Macedo Garitezi <thalesmg@gmail.com>

fix(dsrepl): use correct base dir for ra system stuff

f2268aa

Co-Authored-By: Thales Macedo Garitezi <thalesmg@gmail.com>

fix(dsrepl): properly handle error conditions in generation mgmt

35b18f9

Also update few outdated typespecs. Also make error reasons easier to comprehend.

chore(dsrepl): mark per-node add_generation RPC target obsolete

d39ca41

Also annotate internal exports with comments according with their intended use.

fix(dsrepl): turn memoize into a safer function

a1f5de3

fix(ds-egress): drop pending batch on failures

fe50a17

Before this commit, messages in the current batch will be retried as part of next batch. This could have led to message duplication which is probably not what the user wants by default.

test(ds): stabilize t_message_gc testcase

efac5c6

docs(dsrepl): describe briefly what n_sites is for

a8baff6

chore: whitelist ra to make RPCs w/o BPAPIs

e55e1dd

keynslug force-pushed the ft/EMQX-11756/emqx-ds-replication branch from 082d3f3 to e55e1dd Compare March 20, 2024 12:20

ieQu1 previously approved these changes Mar 20, 2024

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fix(ci): add ra to emqx app dependencies as well

7257fe5

keynslug dismissed ieQu1’s stale review via 7257fe5 March 20, 2024 13:47

keynslug requested a review from ieQu1 March 20, 2024 14:32

ieQu1 approved these changes Mar 20, 2024

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keynslug merged commit e10d43c into emqx:master Mar 20, 2024
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feat(ds): implement raft-based replication #12361

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	Nodes = mria_mnesia:running_nodes(),
	Nodes = emqx:running_nodes(),

-    end.
+    end;
+    handle_info(_Info, State) ->
+        {noreply, State}
+    end.

	State#{shards => Shards, status := ready};
	{ok, State#{shards => Shards, status := ready}};

	DataDir = filename:join([emqx:data_dir(), DB, dsrepl]),
	DataDir = filename:join([emqx_ds:base_dir(), DB, dsrepl]),

feat(ds): implement raft-based replication #12361

feat(ds): implement raft-based replication #12361

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