Replica set: electionTimeoutMillis vs heartbeatTimeoutSecs

[HubertBubert]

Hello,

I have a hard time trying to understand relation between these two.

heartbeatTimeoutSecs is the numer of seconds member waits for a hearbeat from other server.

electionTimeoutMillis is the time limit for detecting that the primary server is unavailable.

So let's say we set values as follows:

- heartbeatTimeoutSecs 10

- electionTimeoutMillis 5000  (we just want to reduce failover time, and this parameter should do the trick)

heartbeatTimeoutSecs value means that member is inaccesible after 10 seconds of waiting. So if member waits for 7 s for a heartbeat from primary that means 

primary is still accesible. So there is no reason for election. But on the other hand we have electionTimeoutMillis which means that after 5s primary is considered unavailable.

So:

is electionTimeoutMillis some kind of heartbeatTimeoutSecs only for primary? In other words, does electionTimeoutMillis overwrite heartbeatTimeoutSecs for primary?

cheers,

Hubert

[Asya Kamsky]

Where did you find "heartbeatTimeoutSecs is the numer of seconds member waits for a hearbeat from other server"?  

The docs say: "Internal use only.  The frequency in milliseconds of the heartbeats."

Asya

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[Asya Kamsky]

Ooops, sorry, cut and paste the wrong line from the docs!

heartbeatTimeoutSecs Number of seconds that the replica set members wait for a successful heartbeat from each other. 

Think of heartbeat timeout as "how long do I wait after sending a heartbeat before I decide it's not coming back".

It applies to each heartbeat, and then failed heartbeats trigger other events, including whether an election may be needed.

So it's not really a heartbeat timeout for primaries, it's a separate _additional_ parameter applying to elections in particular.

Asya

[HubertBubert]

I found it here:

https://docs.mongodb.org/manual/reference/replica-configuration/

The explanation you gave is for parameter: settings.heartbeatIntervalMillisThe, but the question is about settings.heartbeatTimeoutSecs.

Hubert

[HubertBubert]

Ooops, sorry, answered to quickly to your first question.

Back to the topic.

I think exactly the same as you. Before 3.2 only hearbeatTimeoutSecs was available. So every member was incaccesible after this time, including master. So lets say election was triggered after this time.

Now we have electionTimeoutMillis and election will be triggered after this time as documentation says. 

Is the electionTimeoutMillis more important and have higher priority? What happens when these two properties have different values?

thank you,

Hubert

[Henrik Ingo]

Hi Hubert

electionTimeoutMillis is used in MongoDB 3.2, when you use replication
protocol v1 (the default for new installations). As far as I
understand, this is the only setting that impacts how long servers
will wait before failover/election is started.

heartbeatTimeoutSecs was used in replication protocol v0.

Note that rs.config() still holds config values for both protocol
versions, so you can change back and forth if needed.

henrik

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[HubertBubert]

Hello henrik,

I think I don't agree with this statement:

"heartbeatTimeoutSecs was used in replication protocol v0"

I think it is still valid in protocol version 1. At least documentetion does not say a word that this is not used in version 1. So I think it is still used. 

And this makes sense. Becouse there must be a time limit for waiting for hearbeat from another member. And this is the limit.

thank you,

Hubert

[Eric Milkie]

Hi Hubert,

The heartbeatTimeoutSecs is still "valid" in protocol version 1, but it has lost its use as a way to control failover sensitivity.  I can't think of a reason to change the parameter in pv1.  Henric is correct in that the only knob that controls failover sensitivity in pv1 is electionTimeoutMillis.

Hope that helps!

-Eric

[Henrik Ingo]

On 20 Jan 2016 19:14, "HubertBubert" <hly...@gmail.com> wrote:
> And this makes sense. Becouse there must be a time limit for waiting for hearbeat from another member. And this is the limit.
>

There must be a limit, and this is exactly what electionTimeoutMillis does.

I should also note the improved predictability of protocol version v1. In prior versions of Mongodb, there were many different timeouts to trigger failover. (Not all of them configurable.) In v1, you can expect the timeout to be at most electionTimeoutMillis.

Henrik

[HubertBubert]

Hello,

Thank you all for help.

best regards,

Hubert