CP-48676: Reuse pool sessions on slave logins #6258
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Prevent this reusable pool session from being destroyed so that it remains valid. This feature can be toggled with the flag reuse-pool-sessions. This commit has been reworked to use Atomic instead of a mutex to prevent a deadlock as shown in CA-400339. Signed-off-by: Steven Woods <steven.woods@citrix.com>
Add a new flag validate-reusable-pool-session to xapi globs which skips the reusable pool session validity check if it is false. This saves time as we are no longer calling pool.get_all for each session. Signed-off-by: Steven Woods <steven.woods@citrix.com>
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| ignore (Client.Pool.get_all ~rpc ~session_id) ; | ||
| true | ||
| with Api_errors.Server_error (err, _) -> | ||
| info "Invalid session: %s" err ; |
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This should be debug loglevel, or be removed
ocaml/xapi/xapi_session.ml
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| in | ||
| let rec get_session () = | ||
| let session = Atomic.get reusable_pool_session in | ||
| if session <> Ref.null && is_valid_session session then ( |
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I recommend changing this to
| if session <> Ref.null && is_valid_session session then ( | |
| if is_valid session then ( |
And add the null check to is_valid
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The issue is that in the next commit, where we only validate if validate_reusable_pool_session is true, we want to always do the null check but only do validation if the flag is set. We want to skip validation as it saves a lot of time.
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The validation can (and should) encapsulate all this logic
| if session <> Ref.null && is_valid_session session then ( | ||
| if | ||
| session <> Ref.null | ||
| && ((not !Xapi_globs.validate_reusable_pool_session) |
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Please add the flag check to is_valid as well
| @@ -1065,6 +1065,10 @@ let pool_recommendations_dir = ref "/etc/xapi.pool-recommendations.d" | |||
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| let disable_webserver = ref false | |||
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| let reuse_pool_sessions = ref true | |||
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The comment from the PR could go here as a comment to document the semantics.
| ; ( "validate-reusable-pool-session" | ||
| , Arg.Set validate_reusable_pool_session | ||
| , (fun () -> string_of_bool !validate_reusable_pool_session) | ||
| , "Enable the reuse of pool sessions" |
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The documentation is not accurate.
This was first introduced in #5986 but was later reverted as the mutex it introduced caused a deadlock with the DB mutex. This PR has now been reworked to use an Atomic variable, removing the need for the mutex.
This PR allows pool sessions to be reused (if the flag is turned on, which it is by default). This greatly speeds up communications between hosts which is important for repetitive calls e.g. one customer was running get_servertime frequently on all hosts in a pool which was causing problems due to the length of each call.
When we get this reusable session, we can optionally validate the session before using it but this increases the duration of the call, so this is off by default. Excepting a toolstack restart, the reusable session should always be valid as it is excluded from deletion in destroy_db_session