tsdb: Isolation for writes

[gouthamve]

Currently, when we call app.Commit(), we go ahead and start adding samples to the database. But now if there is a concurrent reader that is reading data, it might read some samples from the commit but not all.

This causes issues in Prometheus and has been known to make histograms falter: #1169

This PR is an attempt to fix it: prometheus-junkyard/tsdb#105

[fabxc]

The issue with that PR was mostly that it broke apart our APIs relatively forcefully to make it fit. I once had an attempt around that avoided this and should in theory be a bit more memory friendly. However, that is more than stale as well by now.

…

On Tue, Jan 16, 2018 at 10:51 AM Goutham Veeramachaneni < ***@***.***> wrote: Currently, when we call app.Commit(), we go ahead and start adding samples to the database. But now if there is a concurrent reader that is reading data, it might read some samples from the commit but not all. This causes issues in Prometheus and has been known to make histograms falter: prometheus/prometheus#1169 <#1169> This PR is an attempt to fix it: prometheus-junkyard/tsdb#105 <prometheus-junkyard/tsdb#105> — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <https://github.com/prometheus/tsdb/issues/260>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AEuA8vRFVlieAq1Btt92KE3PwO5nl0ddks5tLHE8gaJpZM4RfgOy> .

[brian-brazil]

Isolation is a fairly core thing for a database, I'd be surprised if it didn't require a similar level of changes as what I proposed.

[fabxc]

Note that the structure changed a fair bit and some things got simplified into a single place since. That alone makes some leakage across the some main interfaces no longer necessary.

…

On Tue, Jan 16, 2018 at 7:01 PM Brian Brazil ***@***.***> wrote: Isolation is a fairly core thing for a database, I'd be surprised if it didn't require a similar level of changes as what I proposed. — You are receiving this because you commented. Reply to this email directly, view it on GitHub <https://github.com/prometheus/tsdb/issues/260#issuecomment-358050792>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AEuA8rhGyOvbhxOMNihKwRYpCfGhwJ-Kks5tLOQUgaJpZM4RfgOy> .

[roidelapluie]

@beorn7 @brian-brazil this can be closed, right?