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https://issues.apache.org/jira/browse/GEODE-9680?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Bill Burcham updated GEODE-9680:
--------------------------------
Description:
The issues described here are present in all versions of Geode (this is not new
to 1.15.0)…
Geode is built on the assumption that views progress linearly in a sequence. If
that sequence ever forks into two or more parallel lines then we have a "split
brain".
In a split brain condition, each of the parallel views are independent. It's as
if you have more than one system running concurrently. It's possible e.g. for
some clients to connect to members of one view and other clients to connect to
members of another view. Updates to members in one view are not seen by members
of a parallel view.
Geode views are produced by a coordinator. As long as only a single coordinator
is running, there is no possibility of a split brain. Split brain arises when
more than one coordinator is producing views at the same time.
Each Geode member (peer) is started with the {{locators}} configuration
parameter. That parameter specifies locator(s) to use to find the (already
running!) coordinator (member) to join with.
When a locator (member) starts, it goes through this sequence to find the
coordinator:
# it first tries to find the coordinator through one of the (other) configured
locators
# if it can't contact any of those, it tries contacting non-locator (cache
server) members it has retrieved from the "view presistence" ({{.dat}}) file
If it hasn't found a coordinator to join with, then the locator may _become_ a
coordinator.
Sometimes this is ok. If no other coordinator is currently running then this
behavior is fine. An example is when an [administrator is starting up a brand
new
cluster|https://geode.apache.org/docs/guide/114/configuring/running/running_the_locator.html].
In that case we want the very first locator we start to become the coordinator.
But there are a number of situations where there may already be another
coordinator running but it cannot be reached:
* if the administrator/operator is starting up a brand new cluster with
multiple locators and…
** maybe Geode is running in a managed environment like Kubernetes and the
locators hostnames are not (yet) resolvable in DNS
** maybe there is a network partition between the starting locators so they
can't communicate
** maybe the existing locators or coordinator are running very slowly or the
network is degraded. This is effectively the same as the network partition just
mentioned
* if a cluster is already running and the administrator/operator wants to
scale it up by starting/adding a new locator Geode is susceptible to that same
network partition issue
* if a cluster is already running and the administrator/operator needs to
restart a locator, e.g. for a rolling upgrade, if none of the locators in the
{{locators}} configuration parameter are reachable (maybe they are not running,
or maybe there is a network partition) and…
** if the "view persistence" {{.dat}} file is missing or deleted
** or if the current set of running Geode members has evolved so far that the
coordinates (host+port) in the {{.dat}} file are completely out of date
In each of those cases, the newly starting locator will become a coordinator
and will start producing views. Now we'll have the old coordinator producing
views at the same time as the new one.
*When this ticket is complete*, Geode will offer a locator startup mode (via
TBD {{LocatorLauncher}} startup parameter) that prevents that locator from
becoming a coordinator. With that mode, it will be possible for an
administrator/operator to avoid many of the problematic scenarios mentioned
above, while retaining the ability to start a first locator which is allowed to
become a coordinator.
For purposes of discussion we'll call the startup mode that allows the locator
to become a coordinator "seed" mode, and we'll call the new startup mode that
prevents the locator from becoming a coordinator before first joining,
"join-only" mode.
To start a brand new cluster, an administrator/operator starts the first
locator in "seed" mode. After that the operator starts all subsequent locators
in "join only" mode. If network partitions occur during startup, those newly
started nodes will exit with a failure status, but will not become coordinators.
To add a locator to a running cluster, an operator starts it in "join only"
mode. The new member will similarly either join with an existing coordinator or
exit with a failure status, thereby avoiding split brains.
When an operator restarts a locator, e.g. during a rolling upgrade, they will
restarted in "join only" mode. If a network partition is encountered, or the
{{.dat}} file is missing or stale, the new locator will exit with a failure
status and split brain will be avoided.
h2.
FAQ
Q: What should happen if a locator is started in seed mode, but it can see
another view member is already acting as coordinator?
A: TBD
Q: How long will join only wait before giving up and exiting?
A: TBD
was:
The issues described here are present in all versions of Geode (this is not new
to 1.15.0)…
Geode is built on the assumption that views progress linearly in a sequence. If
that sequence ever forks into two or more parallel lines then we have a "split
brain".
In a split brain condition, each of the parallel views are independent. It's as
if you have more than one system running concurrently. It's possible e.g. for
some clients to connect to members of one view and other clients to connect to
members of another view. Updates to members in one view are not seen by members
of a parallel view.
Geode views are produced by a coordinator. As long as only a single coordinator
is running, there is no possibility of a split brain. Split brain arises when
more than one coordinator is producing views at the same time.
Each Geode member (peer) is started with the {{locators}} configuration
parameter. That parameter specifies locator(s) to use to find the (already
running!) coordinator (member) to join with.
When a locator (member) starts, it goes through this sequence to find the
coordinator:
# it first tries to find the coordinator through one of the (other) configured
locators
# if it can't contact any of those, it tries contacting non-locator (cache
server) members it has retrieved from the "view presistence" ({{.dat}}) file
If it hasn't found a coordinator to join with, then the locator may _become_ a
coordinator.
Sometimes this is ok. If no other coordinator is currently running then this
behavior is fine. An example is when an [administrator is starting up a brand
new
cluster|https://geode.apache.org/docs/guide/114/configuring/running/running_the_locator.html].
In that case we want the very first locator we start to become the coordinator.
But there are a number of situations where there may already be another
coordinator running but it cannot be reached:
* if the administrator/operator is starting up a brand new cluster with
multiple locators and…
** maybe Geode is running in a managed environment like Kubernetes and the
locators hostnames are not (yet) resolvable in DNS
** maybe there is a network partition between the starting locators so they
can't communicate
** maybe the existing locators or coordinator are running very slowly or the
network is degraded. This is effectively the same as the network partition just
mentioned
* if a cluster is already running and the administrator/operator wants to
scale it up by starting/adding a new locator Geode is susceptible to that same
network partition issue
* if a cluster is already running and the administrator/operator needs to
restart a locator, e.g. for a rolling upgrade, if none of the locators in the
{{locators}} configuration parameter are reachable (maybe they are not running,
or maybe there is a network partition) and…
** if the "view persistence" {{.dat}} file is missing or deleted
** or if the current set of running Geode members has evolved so far that the
coordinates (host+port) in the {{.dat}} file are completely out of date
In each of those cases, the newly starting locator will become a coordinator
and will start producing views. Now we'll have the old coordinator producing
views at the same time as the new one.
*When this ticket is complete*, Geode will offer a locator startup mode (via
TBD {{LocatorLauncher}} startup parameter) that prevents that locator from
becoming a coordinator. With that mode, it will be possible for an
administrator/operator to avoid many of the problematic scenarios mentioned
above, while retaining the ability to start a first locator which is allowed to
become a coordinator.
For purposes of discussion we'll call the startup mode that allows the locator
to become a coordinator "seed" mode, and we'll call the new startup mode that
prevents the locator from becoming a coordinator before first joining,
"join-only" mode.
To start a brand new cluster, an administrator/operator starts the first
locator in "seed" mode. After that the operator starts all subsequent locators
in "join only" mode. If network partitions occur during startup, those newly
started nodes will exit with a failure status, but will not become coordinators.
To add a locator to a running cluster, an operator starts it in "join only"
mode. The new member will similarly either join with an existing coordinator or
exit with a failure status, thereby avoiding split brains.
When an operator restarts a locator, e.g. during a rolling upgrade, they will
restarted in "join only" mode. If a network partition is encountered, or the
{{.dat}} file is missing or stale, the new locator will exit with a failure
status and split brain will be avoided.
> Newly Started/Restarted Locators are Susceptible to Split-Brains
> ----------------------------------------------------------------
>
> Key: GEODE-9680
> URL: https://issues.apache.org/jira/browse/GEODE-9680
> Project: Geode
> Issue Type: Bug
> Components: membership
> Affects Versions: 1.15.0
> Reporter: Bill Burcham
> Priority: Major
> Labels: needsTriage
>
> The issues described here are present in all versions of Geode (this is not
> new to 1.15.0)…
> Geode is built on the assumption that views progress linearly in a sequence.
> If that sequence ever forks into two or more parallel lines then we have a
> "split brain".
> In a split brain condition, each of the parallel views are independent. It's
> as if you have more than one system running concurrently. It's possible e.g.
> for some clients to connect to members of one view and other clients to
> connect to members of another view. Updates to members in one view are not
> seen by members of a parallel view.
> Geode views are produced by a coordinator. As long as only a single
> coordinator is running, there is no possibility of a split brain. Split brain
> arises when more than one coordinator is producing views at the same time.
> Each Geode member (peer) is started with the {{locators}} configuration
> parameter. That parameter specifies locator(s) to use to find the (already
> running!) coordinator (member) to join with.
> When a locator (member) starts, it goes through this sequence to find the
> coordinator:
> # it first tries to find the coordinator through one of the (other)
> configured locators
> # if it can't contact any of those, it tries contacting non-locator (cache
> server) members it has retrieved from the "view presistence" ({{.dat}}) file
> If it hasn't found a coordinator to join with, then the locator may _become_
> a coordinator.
> Sometimes this is ok. If no other coordinator is currently running then this
> behavior is fine. An example is when an [administrator is starting up a brand
> new
> cluster|https://geode.apache.org/docs/guide/114/configuring/running/running_the_locator.html].
> In that case we want the very first locator we start to become the
> coordinator.
> But there are a number of situations where there may already be another
> coordinator running but it cannot be reached:
> * if the administrator/operator is starting up a brand new cluster with
> multiple locators and…
> ** maybe Geode is running in a managed environment like Kubernetes and the
> locators hostnames are not (yet) resolvable in DNS
> ** maybe there is a network partition between the starting locators so they
> can't communicate
> ** maybe the existing locators or coordinator are running very slowly or the
> network is degraded. This is effectively the same as the network partition
> just mentioned
> * if a cluster is already running and the administrator/operator wants to
> scale it up by starting/adding a new locator Geode is susceptible to that
> same network partition issue
> * if a cluster is already running and the administrator/operator needs to
> restart a locator, e.g. for a rolling upgrade, if none of the locators in the
> {{locators}} configuration parameter are reachable (maybe they are not
> running, or maybe there is a network partition) and…
> ** if the "view persistence" {{.dat}} file is missing or deleted
> ** or if the current set of running Geode members has evolved so far that
> the coordinates (host+port) in the {{.dat}} file are completely out of date
> In each of those cases, the newly starting locator will become a coordinator
> and will start producing views. Now we'll have the old coordinator producing
> views at the same time as the new one.
> *When this ticket is complete*, Geode will offer a locator startup mode (via
> TBD {{LocatorLauncher}} startup parameter) that prevents that locator from
> becoming a coordinator. With that mode, it will be possible for an
> administrator/operator to avoid many of the problematic scenarios mentioned
> above, while retaining the ability to start a first locator which is allowed
> to become a coordinator.
> For purposes of discussion we'll call the startup mode that allows the
> locator to become a coordinator "seed" mode, and we'll call the new startup
> mode that prevents the locator from becoming a coordinator before first
> joining, "join-only" mode.
> To start a brand new cluster, an administrator/operator starts the first
> locator in "seed" mode. After that the operator starts all subsequent
> locators in "join only" mode. If network partitions occur during startup,
> those newly started nodes will exit with a failure status, but will not
> become coordinators.
> To add a locator to a running cluster, an operator starts it in "join only"
> mode. The new member will similarly either join with an existing coordinator
> or exit with a failure status, thereby avoiding split brains.
> When an operator restarts a locator, e.g. during a rolling upgrade, they will
> restarted in "join only" mode. If a network partition is encountered, or the
> {{.dat}} file is missing or stale, the new locator will exit with a failure
> status and split brain will be avoided.
> h2.
> FAQ
> Q: What should happen if a locator is started in seed mode, but it can see
> another view member is already acting as coordinator?
> A: TBD
>
> Q: How long will join only wait before giving up and exiting?
> A: TBD
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