Diviner Phabricator User Docs Clustering Introduction

Clustering Introduction
Phabricator User Documentation (Cluster Configuration)

Guide to configuring Phabricator across multiple hosts for availability and performance.


WARNING: This feature is a prototype. Installs should expect a challenging adventure when deploying clusters. In the best of times, configuring a cluster is complex and requires significant operations experience.

Phabricator can be configured to run on multiple hosts with redundant services to improve its availability and scalability, and make disaster recovery much easier.

Clustering is more complex to setup and maintain than running everything on a single host, but greatly reduces the cost of recovering from hardware and network failures.

Each Phabricator service has an array of clustering options that can be configured somewhat independently. Configuring a cluster is inherently complex, and this is an advanced feature aimed at installs with large userbases and experienced operations personnel who need this high degree of flexibility.

The remainder of this document summarizes how to add redundancy to each service and where your efforts are likely to have the greatest impact.

For additional guidance on setting up a cluster, see "Overlaying Services" and "Cluster Recipes" at the bottom of this document.

Clusterable Services

This table provides an overview of clusterable services, their setup complexity, and the rough impact that converting them to run on multiple hosts will have on availability, resistance to data loss, and scalability.

ServiceSetupAvailabilityLoss ResistanceScalability
DaemonsMinimalLowNo RiskLow
SSH ServersMinimalLowNo RiskLow
Web ServersMinimalHighNo RiskModerate
NotificationsMinimalLowNo RiskLow
Fulltext SearchMinimalLowNo RiskLow

See below for a walkthrough of these services in greater detail.

Preparing for Clustering

To begin deploying Phabricator in cluster mode, set up cluster.addresses in your configuration.

This option should contain a list of network address blocks which are considered to be part of the cluster. Hosts in this list are allowed to bend (or even break) some of the security and policy rules when they make requests to other hosts in the cluster, so this list should be as small as possible. See "Cluster Whitelist Security" below for discussion.

If you are deploying hardware in EC2, a reasonable approach is to launch a dedicated Phabricator VPC, whitelist the whole VPC as a Phabricator cluster, and then deploy only Phabricator services into that VPC.

If you have additional auxiliary hosts which run builds and tests via Drydock, you should not include them in the cluster address definition. For more detailed discussion of the Drydock security model, see Drydock User Guide: Security.

Most other clustering features will not work until you define a cluster by configuring cluster.addresses.

Cluster Whitelist Security

When you configure cluster.addresses, you should keep the list of trusted cluster hosts as small as possible. Hosts on this list gain additional capabilities, including these:

Trusted HTTP Headers: Normally, Phabricator distrusts the load balancer HTTP headers X-Forwarded-For and X-Forwarded-Proto because they may be client-controlled and can be set to arbitrary values by an attacker if no load balancer is deployed. In particular, clients can set X-Forwarded-For to any value and spoof traffic from arbitrary remotes.

These headers are trusted when they are received from a host on the cluster address whitelist. This allows requests from cluster loadbalancers to be interpreted correctly by default without requiring additional custom code or configuration.

Intracluster HTTP: Requests from cluster hosts are not required to use HTTPS, even if security.require-https is enabled, because it is common to terminate HTTPS on load balancers and use plain HTTP for requests within a cluster.

Special Authentication Mechanisms: Cluster hosts are allowed to connect to other cluster hosts with "root credentials", and to impersonate any user account.

The use of root credentials is required because the daemons must be able to bypass policies in order to function properly: they need to send mail about private conversations and import commits in private repositories.

The ability to impersonate users is required because SSH nodes must receive, interpret, modify, and forward SSH traffic. They can not use the original credentials to do this because SSH authentication is asymmetric and they do not have the user's private key. Instead, they use root credentials and impersonate the user within the cluster.

These mechanisms are still authenticated (and use asymmetric keys, like SSH does), so access to a host in the cluster address block does not mean that an attacker can immediately compromise the cluster. However, an over-broad cluster address whitelist may give an attacker who gains some access additional tools to escalate access.

Note that if an attacker gains access to an actual cluster host, these extra powers are largely moot. Most cluster hosts must be able to connect to the master database to function properly, so the attacker will just do that and freely read or modify whatever data they want.

Cluster: Databases

Configuring multiple database hosts is moderately complex, but normally has the highest impact on availability and resistance to data loss. This is usually the most important service to make redundant if your focus is on availability and disaster recovery.

Configuring replicas allows Phabricator to run in read-only mode if you lose the master and to quickly promote the replica as a replacement.

For details, see Cluster: Databases.

Cluster: Repositories

Configuring multiple repository hosts is complex, but is required before you can add multiple daemon or web hosts.

Repository replicas are important for availability if you host repositories on Phabricator, but less important if you host repositories elsewhere (instead, you should focus on making that service more available).

The distributed nature of Git and Mercurial tend to mean that they are naturally somewhat resistant to data loss: every clone of a repository includes the entire history.

Repositories may become a scalability bottleneck, although this is rare unless your install has an unusually heavy repository read volume. Slow clones/fetches may hint at a repository capacity problem. Adding more repository hosts will provide an approximately linear increase in capacity.

For details, see Cluster: Repositories.

Cluster: Daemons

Configuring multiple daemon hosts is straightforward, but you must configure repositories first.

With daemons running on multiple hosts you can transparently survive the loss of any subset of hosts without an interruption to daemon services, as long as at least one host remains alive. Daemons are stateless, so spreading daemons across multiple hosts provides no resistance to data loss.

Daemons can become a bottleneck, particularly if your install sees a large volume of write traffic to repositories. If the daemon task queue has a backlog, that hints at a capacity problem. If existing hosts have unused resources, increase phd.taskmasters until they are fully utilized. From there, adding more daemon hosts will provide an approximately linear increase in capacity.

For details, see Cluster: Daemons.

Cluster: SSH Servers

Configuring multiple SSH hosts is straightforward, but you must configure repositories first.

With multiple SSH hosts you can transparently survive the loss of any subset of hosts without interruption to repository services, as long as at last one host remains alive. SSH services are stateless, so putting multiple hosts in service provides no resistance to data loss because no data is at risk.

SSH hosts are very rarely a scalability bottleneck.

For details, see Cluster: SSH Servers.

Cluster: Web Servers

Configuring multiple web hosts is straightforward, but you must configure repositories first.

With multiple web hosts you can transparently survive the loss of any subset of hosts as long as at least one host remains alive. Web services are stateless, so putting multiple hosts in service provides no resistance to data loss because no data is at risk.

Web hosts can become a bottleneck, particularly if you have a workload that is heavily focused on reads from the web UI (like a public install with many anonymous users). Slow responses to web requests may hint at a web capacity problem. Adding more hosts will provide an approximately linear increase in capacity.

For details, see Cluster: Web Servers.

Cluster: Notifications

Configuring multiple notification hosts is simple and has no pre-requisites.

With multiple notification hosts, you can survive the loss of any subset of hosts as long as at least one host remains alive. Service may be briefly disrupted directly after the incident which destroys the other hosts.

Notifications are noncritical, so this normally has little practical impact on service availability. Notifications are also stateless, so clustering this service provides no resistance to data loss because no data is at risk.

Notification delivery normally requires very few resources, so adding more hosts is unlikely to have much impact on scalability.

For details, see Cluster: Notifications.

Cluster: Fulltext Search

Configuring search services is relatively simple and has no pre-requisites.

By default, Phabricator uses MySQL as a fulltext search engine, so deploying multiple database hosts will effectively also deploy multiple fulltext search hosts.

Search indexes can be completely rebuilt from the database, so there is no risk of data loss no matter how fulltext search is configured.

For details, see Cluster: Search.

Overlaying Services

Although hosts can run a single dedicated service type, certain groups of services work well together. Phabricator clusters usually do not need to be very large, so deploying a small number of hosts with multiple services is a good place to start.

In planning a cluster, consider these blended host types:

Everything: Run HTTP, SSH, MySQL, notifications, repositories and daemons on a single host. This is the starting point for single-node setups, and usually also the best configuration when adding the second node.

Everything Except Databases: Run HTTP, SSH, notifications, repositories and daemons on one host, and MySQL on a different host. MySQL uses many of the same resources that other services use. It's also simpler to separate than other services, and tends to benefit the most from dedicated hardware.

Repositories and Daemons: Run repositories and daemons on the same host. Repository hosts must run daemons, and it normally makes sense to completely overlay repositories and daemons. These services tend to use different resources (repositories are heavier on I/O and lighter on CPU/RAM; daemons are heavier on CPU/RAM and lighter on I/O).

Repositories and daemons are also both less latency sensitive than other service types, so there's a wider margin of error for under provisioning them before performance is noticeably affected.

These nodes tend to use system resources in a balanced way. Individual nodes in this class do not need to be particularly powerful.

Frontend Servers: Run HTTP and SSH on the same host. These are easy to set up, stateless, and you can scale the pool up or down easily to meet demand. Routing both types of ingress traffic through the same initial tier can simplify load balancing.

These nodes tend to need relatively little RAM.

Cluster Recipes

This section provides some guidance on reasonable ways to scale up a cluster.

The smallest possible cluster is two hosts. Run everything (web, ssh, database, notifications, repositories, and daemons) on each host. One host will serve as the master; the other will serve as a replica.

Ideally, you should physically separate these hosts to reduce the chance that a natural disaster or infrastructure disruption could disable or destroy both hosts at the same time.

From here, you can choose how you expand the cluster.

To improve scalability and performance, separate loaded services onto dedicated hosts and then add more hosts of that type to increase capacity. If you have a two-node cluster, the best way to improve scalability by adding one host is likely to separate the master database onto its own host.

Note that increasing scale may decrease availability by leaving you with too little capacity after a failure. If you have three hosts handling traffic and one datacenter fails, too much traffic may be sent to the single remaining host in the surviving datacenter. You can hedge against this by mirroring new hosts in other datacenters (for example, also separate the replica database onto its own host).

After separating databases, separating repository + daemon nodes is likely the next step to consider.

To improve availability, add another copy of everything you run in one datacenter to a new datacenter. For example, if you have a two-node cluster, the best way to improve availability is to run everything on a third host in a third datacenter. If you have a 6-node cluster with a web node, a database node and a repo + daemon node in two datacenters, add 3 more nodes to create a copy of each node in a third datacenter.

You can continue adding hosts until you run out of hosts.

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