A stack is a efficient copy of your archive, stored nearby or far away.

Stacks are designed to be incrementally synchronized with your archive. So as your archive changes; new layers are added and merged layers replaced (or not, your choice).

A stack can later be used to restore detail, repair layers, or even recover the entire archive.

Stack data is stored in a Stack Container. Curently there are (fundamentally) two container types: a writable filesytem or an Amazon Web Services (AWS) S3 cloud object storage bucket. Additional container types are being developed.

A stack is bound to a single archive and can only exchange data with that one archive.

An archive can have multiple stacks. But all of the stacks will be copies of the same archive. For example, an archive of extremely sensative data might have a local (filesystem) stack for immediate duplication of captured files, along with a second long-term stack maintained on a remote cloud data service that only gets updated once or twice a month.

Stacks are organized into layers, just like your archive.

A key concept of stacks is a "slice." A slice is a set of layers in the archive that paired with an equivelent set of layers in the stack. In this context, "equivelent" means those layers represent the same set of captured changes.

In the beginning, when the layers in the archive are first transferred to the stack, there's a one-to-one pairing of layers, which each layer pair forming a single slice.



As layers are merged, however, this relationship changes. For example, if you merge layers 3-5 in the archive, the new singe (merged) layer now represents the same captured data as layers 3-5 in the stack.



In the above example, layer 30 in the archive can replace layers 31-33 in the stack. When that happens, the stack size is reduced by the same amount of storage receovered in the archive.

But the reverse is also possible. Archive layer 30 can be replaced with stack layers 31-33, restoring the intermedaite changes that were lost during in the merge.

Only whole slices can be transferred between the archive and the stack. This is an important concept, and the reason should be obvious. The single merged layer (30) in the archive represents the same set of item changes as layers 31-33 in the stack, just without the intermediate detail.

Moving on...

The post on creating stacks will show you how to seed the stack with the initial set of layers. Then read the post on updating stacks, and then the post on restoring slices and archives.