open-consul/vendor/github.com/hashicorp/go-memdb/README.md

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# go-memdb [![CircleCI](https://circleci.com/gh/hashicorp/go-memdb/tree/master.svg?style=svg)](https://circleci.com/gh/hashicorp/go-memdb/tree/master)
Provides the `memdb` package that implements a simple in-memory database
built on immutable radix trees. The database provides Atomicity, Consistency
and Isolation from ACID. Being that it is in-memory, it does not provide durability.
New ACLs (#4791) This PR is almost a complete rewrite of the ACL system within Consul. It brings the features more in line with other HashiCorp products. Obviously there is quite a bit left to do here but most of it is related docs, testing and finishing the last few commands in the CLI. I will update the PR description and check off the todos as I finish them over the next few days/week. Description At a high level this PR is mainly to split ACL tokens from Policies and to split the concepts of Authorization from Identities. A lot of this PR is mostly just to support CRUD operations on ACLTokens and ACLPolicies. These in and of themselves are not particularly interesting. The bigger conceptual changes are in how tokens get resolved, how backwards compatibility is handled and the separation of policy from identity which could lead the way to allowing for alternative identity providers. On the surface and with a new cluster the ACL system will look very similar to that of Nomads. Both have tokens and policies. Both have local tokens. The ACL management APIs for both are very similar. I even ripped off Nomad's ACL bootstrap resetting procedure. There are a few key differences though. Nomad requires token and policy replication where Consul only requires policy replication with token replication being opt-in. In Consul local tokens only work with token replication being enabled though. All policies in Nomad are globally applicable. In Consul all policies are stored and replicated globally but can be scoped to a subset of the datacenters. This allows for more granular access management. Unlike Nomad, Consul has legacy baggage in the form of the original ACL system. The ramifications of this are: A server running the new system must still support other clients using the legacy system. A client running the new system must be able to use the legacy RPCs when the servers in its datacenter are running the legacy system. The primary ACL DC's servers running in legacy mode needs to be a gate that keeps everything else in the entire multi-DC cluster running in legacy mode. So not only does this PR implement the new ACL system but has a legacy mode built in for when the cluster isn't ready for new ACLs. Also detecting that new ACLs can be used is automatic and requires no configuration on the part of administrators. This process is detailed more in the "Transitioning from Legacy to New ACL Mode" section below.
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The database is instantiated with a schema that specifies the tables and indices
that exist and allows transactions to be executed.
The database provides the following:
* Multi-Version Concurrency Control (MVCC) - By leveraging immutable radix trees
the database is able to support any number of concurrent readers without locking,
and allows a writer to make progress.
* Transaction Support - The database allows for rich transactions, in which multiple
objects are inserted, updated or deleted. The transactions can span multiple tables,
and are applied atomically. The database provides atomicity and isolation in ACID
terminology, such that until commit the updates are not visible.
* Rich Indexing - Tables can support any number of indexes, which can be simple like
a single field index, or more advanced compound field indexes. Certain types like
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UUID can be efficiently compressed from strings into byte indexes for reduced
storage requirements.
* Watches - Callers can populate a watch set as part of a query, which can be used to
detect when a modification has been made to the database which affects the query
results. This lets callers easily watch for changes in the database in a very general
way.
For the underlying immutable radix trees, see [go-immutable-radix](https://github.com/hashicorp/go-immutable-radix).
Documentation
=============
The full documentation is available on [Godoc](http://godoc.org/github.com/hashicorp/go-memdb).
Example
=======
Below is a simple example of usage
```go
// Create a sample struct
type Person struct {
Email string
Name string
Age int
}
// Create the DB schema
schema := &memdb.DBSchema{
Tables: map[string]*memdb.TableSchema{
"person": &memdb.TableSchema{
Name: "person",
Indexes: map[string]*memdb.IndexSchema{
"id": &memdb.IndexSchema{
Name: "id",
Unique: true,
Indexer: &memdb.StringFieldIndex{Field: "Email"},
},
},
},
},
}
// Create a new data base
db, err := memdb.NewMemDB(schema)
if err != nil {
panic(err)
}
// Create a write transaction
txn := db.Txn(true)
// Insert a new person
p := &Person{"joe@aol.com", "Joe", 30}
if err := txn.Insert("person", p); err != nil {
panic(err)
}
// Commit the transaction
txn.Commit()
// Create read-only transaction
txn = db.Txn(false)
defer txn.Abort()
// Lookup by email
raw, err := txn.First("person", "id", "joe@aol.com")
if err != nil {
panic(err)
}
// Say hi!
fmt.Printf("Hello %s!", raw.(*Person).Name)
```