dCache, as distributed storage software, can provide a coherent service using multiple computers or nodes (the two terms are used interchangeable). Although dCache can provide a complete storage solution on a single computer, one of its strengths is the ability to scale by spreading the work over multiple nodes.
A cell is dCache’s most fundamental executable building block. Even a small dCache deployment will have many cells running. Each cell has a specific task to perform and most will interact with other cells to achieve it.
Cells can be grouped into common types; for example, pools, doors. Cells of the same type behave in a similar fashion and have higher-level behaviour (such as storing files, making files available). Later chapters will describe these different cell types and how they interact in more detail.
There are only a few cells where (at most) only a single instance is required. The majority of cells within a dCache instance can have multiple instances and dCache is designed to allow load-balancing over these cells.
A domain is a container for running cells. Each domain runs in its own Java Virtual Machine (JVM) instance, which it cannot share with any other domain. In essence, a domain is a JVM with the additional functionality necessary to run cells (such as system administration and inter-cell communication). This also implies, that a node’s resources, such as memory, available CPU and network bandwidth, are shared among several domains running on the same node.
dCache comes with a set of domain definitions, each specifying a useful set of cells to run within that domain to achieve a certain goal. These goals include storing data, providing a front-end to the storage, recording file names, and so on. The list of cells to run within these domains are recommended deployments: the vast majority of dCache deployments do not need to alter these lists.
A node is free to run multiple domains, provided there’s no conflicting requirement from the domains for exclusive access to hardware. A node may run a single domain; but, typically a node will run multiple domains. The choice of which domains to run on which nodes will depend on expected load of the dCache instance and on the available hardware. If this sounds daunting, don’t worry: starting and stopping a domain is easy and migrating a domain from one node to another is often as easy as stopping the domain on one node and starting it on another.
dCache is scalable storage software. This means that (in most cases) the performance of dCache can be improved by introducing new hardware. Depending on the performance issue, the new hardware may be used by hosting a domain migrated from a overloaded node, or by running an additional instance of a domain to allow load-balancing.
Most cells communicate in such a way that they don’t rely on in which domain they are running. This allows a site to move cells from one domain to another or to create new domain definitions with some subset of available cells. Although this is possible, it is rare that redefining domains or defining new domains is necessary. Starting or stopping domains is usually sufficient for managing load.