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Maintain deterministic order of CLUSTER SHARDS response #411
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Maintain deterministic order of CLUSTER SHARDS response. Currently we don't maintain the shards/masters in sorted fashion and hence we get the order of CLUSTER SHARDS response non-deterministic on different nodes. Maintain the sorted Masters list of pointers, similar to replicas, and get rid of <shards, list<nodes>> dict which is not suitable for sorting. Add TOPOLOGY argument to get the deterministic response which would remove the replication offset and node health status from cluster shards response. Sort the nodes based on the node Id. Use it in proc `cluster_config_consistent` for the test coverage and sanity purpose. Signed-off-by: Ram Prasad Voleti <ramvolet@amazon.com>
Codecov ReportAttention: Patch coverage is
Additional details and impacted files@@ Coverage Diff @@
## unstable #411 +/- ##
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- Coverage 68.47% 68.45% -0.02%
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Files 109 109
Lines 61669 61670 +1
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- Hits 42225 42216 -9
- Misses 19444 19454 +10
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Thanks @VoletiRam for the PR. There were discussion around using @valkey-io/core-team Please take a look. |
list *clusterGetNodesInMyShard(clusterNode *node) { | ||
sds s = sdsnewlen(node->shard_id, CLUSTER_NAMELEN); | ||
dictEntry *de = dictFind(server.cluster->shards,s); | ||
sdsfree(s); | ||
return (de != NULL) ? dictGetVal(de) : NULL; | ||
clusterNode *master = clusterNodeGetMaster(node); | ||
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list *l = listCreate(); | ||
listAddNodeTail(l, master); | ||
for (int i = 0; i < master->numslaves; i++) { | ||
listAddNodeTail(l, master->slaves[i]); | ||
} | ||
return l; | ||
} |
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This operation becomes O(N) and some additional memory allocation. However, I'm not bothered a lot about it since the code flow is not on the hot path.
Just want to confirm with you @VoletiRam If addling the new parameter "topology" for cluster shards goal is that from every client's view, the output of the 2 masters and 2 replicas nodes is always: 127.0.0.1:6321> cluster shards topology
BWT, I am reviewing this PR codes, Thanks |
@@ -63,6 +63,8 @@ void clusterUpdateState(void); | |||
int clusterNodeCoversSlot(clusterNode *n, int slot); | |||
list *clusterGetNodesInMyShard(clusterNode *node); | |||
int clusterNodeAddSlave(clusterNode *master, clusterNode *slave); | |||
int clusterNodeAddMaster(clusterNode *master); |
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I actually don't like this abstraction. I would like it to be conceptually possible for a shard to exist without a master. @PingXie Thoughts?
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Could you explain a bit more on how such situation can arise?
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I prefer updating this function name as int clusterNodeAddToMasters(clusterNode *node);
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I would like it to be conceptually possible for a shard to exist without a master.
This abstraction seems fine from a quick look. Basically, the contract is to keep track of all the primaries in the cluster in a deterministic order. Even if a primary-less shard could exist, the contract would be upheld regardless I think.
This also reminds me that I will need to create a PR to remove all references in code to "m/s".
I will take a closer look at this PR after merging #245
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Thank you for taking a look. Will wait until you review the PR.
@hwware The ordering is based on primary node id lexicographically irrespective of the |
Thank you @hwware. As @hpatro pointed, the view will be same for both |
void addShardReplyForClusterShards(client *c, list *nodes) { | ||
serverAssert(listLength(nodes) > 0); | ||
clusterNode *n = listNodeValue(listFirst(nodes)); | ||
void addShardReplyForClusterShards(client *c, clusterNode* n, int topology) { |
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minor: clusterNode* n
-> clusterNode* primary
to clarify that this function expects to receive a primary node
Does it filter out failed and loading nodes? It would be ideal if the client's topology map could be solely based on the results of this command, eliminating the need for subsequent checks on the nodes' status. |
Thanks for your words. Then according to this rule, sorted by primary node id lexicographically, all clients should get the same view from any node. (At least primary output is same.) |
@barshaul We are only filtering out fields that contribute to non-deterministic output but not the node's information based on their health status. I think the ask was to eliminate volatile fields that can vary across the clients, at least not clear from discussion in #114. We can help filter out node's information if everyone agrees. |
(c->argc == 2 || c->argc == 3)) | ||
{ | ||
/* CLUSTER SHARDS [TOPOLOGY] */ | ||
int topology = 1; |
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Usually, I set this kind of bool variable default value as 0 (here the variable more close to a bool variable).
But it is not a big issue I think
@@ -4,15 +4,20 @@ | |||
"complexity": "O(N) where N is the total number of cluster nodes", | |||
"group": "cluster", | |||
"since": "7.0.0", | |||
"arity": 2, | |||
"arity": -2, | |||
"container": "CLUSTER", | |||
"function": "clusterCommand", | |||
"command_flags": [ |
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Because you add one more argument here, you need add one item "history" here, for example
"history": [
[
"2.8.0",
"Added the -2 reply."
]
],
void clusterCommandShards(client *c) { | ||
addReplyArrayLen(c, dictSize(server.cluster->shards)); | ||
void clusterCommandShards(client *c, int topology) { | ||
serverAssert(server.cluster->nummasters > 0); |
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I do not think server.cluster->nummasters > 0 checking is must. 0 should work, how do you think?
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Thank you very much for reviewing the changes. Will address the suggestions once everyone finishes reviewing the PR.
Few questions which we need to reach consensus on:
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Thank you @hpatro for raising the questions that need consensus. I want to add couple of questions as well. I am checking few scenarios with 2 primaries - 2 replicas in a 4 node cluster with slot coverage on primaries.
With my implementation, the slot coverage empty issue in ##5 can be solved as we go over each master from masters list and print corresponding slots, but it will still show old master in the response with empty slots and fail health status unless we decide to filter out either master node if marked fail or master with no slot coverage. Please share your opinion. |
Not sure if I understand your question. Can you elaborate?
Based on the use case as described in #114 , including
I don't think 2-shard deployments are legit given the current design/implementation. We need to officially support 2-shard clusters first and then it makes sense to discuss the output of @zuiderkwast should we resurrect the "voting replicas" discussion? redis/redis#12390 |
If all clients would prefer using
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I do not think we should deprecate CLUSTER SHARDS command. Client need to remember one more command and Thus my suggestion is: |
In fact. 2 primaries - 2 replicas cluster, 2 primaries - 0 replicas cluster, 2 primaries - 4 replicas cluster are totally different. Case 1: 2 primaries - 0 replicas cluster. If client set cluster-require-full-coverage as no in conf file, cluster still work even one primary node fail. Case 2: 2 primaries - 2 replicas cluster: If any primary fails, no vote happens, and replica can failover immediately Case 3: 2 primaries - 4 replicas cluster: vote happen if any primary fails So i agree with Ping, let us first support 2-shard clusters first then discuss the output of cluster shards [topology] |
@PingXie Whether replicas can vote or whether the cluster has quorum to perform failovers, or even what kind of consensus algorithm is used, should be irrelevant to the clients. (It's even possible to have some external watchdog that performs manual failover.) So let's decouple those discussions from this PR?
I don't think clients should make their own decisions about the health of nodes. That's something the cluster does for them. The clients should only be concerned with routing according to what the cluster tells them. For this, there's no need to include shards without slots. Maybe it's better to exclude them, because such nodes are usually going to be taken down or are just being set up and not really ready to be used for pubsub and other stuff clients may want to send to them. To summarize: I think CLUSTER SHARDS TOPOLOGY should return no more info that what's included in CLUSTER SLOTS. (Just on a different format.)
I agree with @hwware about this. If clients have started using CLUSTER SHARDS, we can let them do that. Let's not break it. |
If we accept this premise, I think we should consider that maybe we are trying to force
It seems like we are saying that clients just shouldn't care about all the extra data provided by The asks from @barshaul are basically, "I don't want any more information, I just want to know what slots are healthy and able to be served from". That is what So, we can make |
Yes (1) was what I meant, but I wasn't completely aware of the background and details. It seems like the main point of this new CLUSTER SHARDS variant is that it's deterministic, so a you (or a test case) can check that the nodes' views of the cluster is consistent. This isn't the use case for client slot routing. It's rather a use case for test cases and for admins, to check that the cluster converges after adding/removing nodes, slot migrations, etc. If it's deterministic for a healthy cluster even with health info included, then I'm not going to argue against it. It can be used by clients too, just to save some bytes, but if some clients feels they want more info, they'll just use the full version of the command, or CLUSTER NODES. That can't be helped. So I guess the question should be: How common or important is it for cluster admins to check that a cluster converges in this way? (In our own test framework we can solve it in some other way if it's just for us.) |
I've done a fair amount of "diff" between various cluster outputs, and usually have to do some pre-processing to make sure they agree. It would be nice if the node ordering was the same in that case. You could then trivially ignore the fields that are known to be slightly different (replication offset). |
To make my suggestion about cluster slots more concrete, I'm proposing a change so that the response of cluster slots becomes:
Besides that, it behaves the exact same cluster |
@madolson I don't think the reason clients haven't adopted CLUSTER SHARDS (added in 7.0) is that it's hard to parse. The reason is rather that clients want to be backward compatible and support old Redis versions. If we add CLUSTER SLOTS PACKED, it will have the same problem: Clients can only use it if they know the server supports it, and then they still need a fallback for versions that don't. Once Redis 6.2 and all Redis 6-compatible services are EOL (or about the time of Valkey 9 is released), then all deployments support CLUSTER SHARDS and then we can start expecting clients to switch to CLUSTER SHARDS. |
I agree! People don't want to use the I suppose there is another option. If we implement a
I don't agree this will happen. Lots of people will continue to use old versions because they will be supported. |
Regarding this PR: Can we just settle with sorting what can be sorted in CLUSTER SHARDS? No new argument. That's my vote. Then we document what needs to be ignored when comparing the result from two different nodes. That means a doc PR. |
@VoletiRam @hpatro Can you review what Victor posted in the previous message. Instead of adding a new new command, let's just make the existing version deterministically ordered but not make any changes to arguments. |
@VoletiRam As discussed, could we limit the code change(s) to ordering and cleanup the subcommand. The data can be compared as string for topology update easily if it's primary only cluster(s) or node(s) with load once/use data where replication offset doesn't change often. However, client would like to be generic. We should move the conversation to a separate thread regarding improving |
Maintain deterministic order of CLUSTER SHARDS response. Currently we don't maintain the shards/masters in sorted fashion and hence we get the order of CLUSTER SHARDS response non-deterministic on different nodes. Maintain the sorted Masters list of pointers, similar to replicas, and replace the current <shards, list[nodes]> dict which is not suitable for sorting. Add the
TOPOLOGY
argument to get the deterministic response which would remove the replication offset and node health status from cluster shards response. Sort the masters based on the node Id. Include the new CLUSTER SHARDS TOPOLOGY command in the cluster_config_consistent procedure to ensure thorough test coverage and conduct a sanity check on cluster consistency.Example response of
CLUSTER SHARDS TOPOLOGY
in a 2 primaries 2 replicas cluster.Response from Primary 1:
Response from Primary 2:
Ref: #114