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Highly Available dCache Services

This document describes how to configure dCache servics in a high availability mode, avoiding single points of failure and thus enabling rolling upgrades and in some cases horizontal scalability.

For this purpose, dCache services may be grouped into four categories:

  • Pools

  • Doors

  • Central services critical for file access

  • Central services not criticical for file access

Services in each of these groups are described separately in the following.

Before considering redundancy of high level services, it is essential that the low level communication infrastructure used by dCache is redundant. This involves setting up a ZooKeeper cluster of at least three nodes, as well as using a multipath topology for Cell Message passing.

Pools

Pools contain the data files stored in dCache. If a pool is offline, any file stored on this pool and not available on any other pool or on tape will be inaccessible. Upload of new files on the other hand is possible as long as at least one write pool is available.

The only way to avoid that files become inacessible when a pool is offline is to replicate the files onto a second pool. One common way to do this is using the resilience service. That service is described in a separate document.

The development team is working on means to expose the same pool storage through several pool cells, e.g. when using a cluster file system or a third party object store as the data backend. We can, however, not provide any guarantees when that work will be completed.

Doors

Doors in dCache are the enduser endpoints. These expose the files in dCache through one of several supported protocols, e.g. FTP, HTTP, or NFS.

dCache allows an arbitrary number of instances of a door, but these will all appear as separate endpoints to the enduser. There are several possibilities to balance load over these endpoints and to allow rolling upgrades:

  • Use the srm service. The SRM protocol acts as a redirector for transfers and the implementation in dCache provides means of balancing load over several doors as well as drain particular doors to allow them to be upgraded.

  • Let clients discover endpoints through an information service. If clients can discover endpoints through an information service such as BDII, such clients can load balance over available endpoints. dCache doors provides means to dynamically unpublish themselves from the information service and thus to drain a door.

  • Use DNS load balancing. The endpoints are assigned a common DNS name. This requires that clients support balancing the load over the resolved IP addresses or some other means to make clients choose a random endpoint. To drain a door one would remove that door from the DNS record.

  • Use an external load balancing proxy. One can configure an transparent proxy in front of the dCache doors to balance the load over all avaiable endpoints. Such a proxy may be implemented in a load balancing switch or in software such as HA Proxy.

srm

The SRM implementation in dCache is technically a door, even though it merely redirects a client to one of the other endpoints for the actual transfer. In dCache 2.16, the SRM implementation was split into two separate services: The frontend service technically consitutes the door and is called srm. The backend service is called srmmanager and is treated as a central dCache service.

The frontend srm service can be scaled like any other door. Each instance will appear as a separate endpoint and the instances will typically talk to the same srmmanager backend. The same techniques as for other doors may be used to make these appear as a single endpoint to the enduser.

Critical Central Services

Several services are critical for file transfers, meaning if any of these services is unavailable, file transfers may be affected. The way to avoid those to become a single point of failure, and to allow rolling upgrades is more or less the same for all services.

The central concept is that of separating the logical service from the physical service instance. While a physical service instance is always a cell, the logical service is merely an unqualified logical address that any of the physical instances may respond to. As long as other services use the logical service address, any of the physical instances can be unavailable without loss of service.

In the internal dCache messaging system, two types of logical addresses: Topics and queues. While physical cell addresses are fully qualified and contain both the cell name and the dCache domain name, topics and queues lack the domain name suffix (although they internally are represented with an @local suffix). E.g. PnfsManager@namespaceDomain is a fully qualified cell address and identifies a specific cell instance, while PnfsManager is a logical address.

In dCache 2.16, many – although not all – services are replicable. A replicable service is one that supports the above separation between a logical address and a physical address and which supports having several physical instances use the same logical address. One may recognize replicable services by using the dcache services command or by inspecting the default properties for the service and the value of the corresponding cell.replicable property.

To drain a particular instance, one typically removes the cell message route to that instance, effectively decoupling the instance from the logical service name. This is done in the System cell of the domain hosting the instance. E.g. in the admin shell:

\c System@namespaceDomain
route delete PnfsManager PnfsManager@namespaceDomain

After this, no new requests will reach that pnfsmanager instance and once the service is idle it may be shut down.

The following is a list of critical replicable services in dCache 2.16.

spacemanager

Space manager is fully replicable. Several instances must share the same database as requests from doors will be load balanced over all physical instances. The configuration should be synchronized such that all instances are configured the same way.

pinmanager

Pin manager is fully replicable. Several instances must share the same database as requests from doors will be load balanced over all physical instances. The configuration should be synchronized such that all instances are configured the same way.

srmmanager

SRM manager is fully replicable. Several instance must have separate databases. The configuration should be synchronized such that with the exception of the database settings all instances are configured the same way.

Requests are load balanced over all physical instances, but since the SRM protocol is stateful, stateful requests will be tagged by an instance identifier. Each instance registers itself in ZooKeeper and srm frontends forward tagged requests to the corresponding backend instance. When querying requests by token in the backend, one has to strip the tag from the token. E.g. a client may see a token as fb1991c5:-1093540442, where fb1991c5 is a backend indentifier and -1093540442 is the backend token. One may map the backend identifier to an instance through ZooKeeper:

\c System
zk get /dcache/srm/backends/fb1991c5

It is possible to have several logical srmmanager instances, e.g. one per VO. Each logical instance may have one or more physical instances. This may be useful to provide VO specific configuration. In such a setup one would have VO specific srm frontends, each configured to talk to a specific logical backend (which in turn may be implemented by several physical instances).

pnfsmanager

PNFS manager is fully replicable. Several instances must share the same database as requests from doors will be load balanced over all physical instances. The configuration should be synchronized such that all instances are configured the same way.

gplazma

gPlazma is fully replicable. The configuration should be synchronized such that all instances are configured the same way.

Non-critical Central Services

Non-critical services will not directly affect transfers in case of unavailabilty. As such, it may be unnecessary to replicate these services if the primary interest is rolling upgrades.

The following is a list of non-critical replicable services in dCache 2.16.

admin

This is a door and one can have multiple instances of admin like any other door.

httpd

This is a door and one can have multiple instances of httpd like any other door.

info

This service is replicable and one can have multiple instances of info all sharing the same logical service name. Each instance will collect the same information. Requests from httpd will be load balanced over available instances of info.

topo

This service is replicable and one can have multiple instances of topo all sharing the same logical service name. Each instance will collect the same information. Requests will be load balanced over available instances of topo.

statistics

This service collects nightly statistics about available pools. One can have multiple instances of statistics and each instance will collect the same information.