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https://github.com/alex-shapiro/ditto

CRDTs for common data structures like maps, vecs, sets, text, and JSON
https://github.com/alex-shapiro/ditto

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CRDTs for common data structures like maps, vecs, sets, text, and JSON

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README 

        
# Ditto



[![Build Status](https://travis-ci.org/alex-shapiro/ditto.svg?branch=master)](https://travis-ci.org/alex-shapiro/ditto)



Ditto is a library for using [CRDTs](https://en.wikipedia.org/wiki/Conflict-free_replicated_data_type),

or conflict-free replicated data types. CRDTs are data structures

which can be replicated across multiple sites, edited concurrently,

and merged together without leading to conflicts. Ditto provides

a number of commonly used data types:



* **Register\:** A replaceable value

* **Counter:** An i64 value that increments

* **Set\:** A HashSet-like collection of unique values

* **Map\:** A HashMap-like collection of key-value pairs

* **List\:** A Vec-like ordered sequence of elements

* **Text:** A String-like container for mutable text

* **Json:** A JSON value



Ditto's goal is to be fast, correct, and easy to use. If you have any

questions, suggestions, or other feedback, feel free to open an issue

or a pull request or contact the Ditto developers directly.



## Example



```rust

extern crate ditto;

extern crate serde_json;

use ditto::List;



fn main() {

    // Create a List CRDT. The site that creates the CRDT

    // is automatically assigned id 1.

    let mut list1 = List::from(vec![100,200,300]);



    // Send the list's state over a network to a second site with id 2.

    let encoded_state = serde_json::to_string(&list1.state()).unwrap();

    let decoded_state = serde_json::from_str(&encoded_state).unwrap();

    let mut list2 = List::from_state(decoded_state, Some(2)).unwrap();



    // Edit the list concurrently at both the first and second site.

    // Whenever you edit a CRDT, you receive an op that can be sent

    // to other sites.

    let op1 = list1.insert(0, 400).unwrap();

    let op2 = list2.remove(0).1.unwrap();



    // Each site sends its op to the other site for execution.

    // The encoding and decoding has been left out for brevity.

    list1.execute_op(op2);

    list2.execute_op(op1);



    // Now both sites have the same value:

    assert_eq!(list1.state(), list2.state());

    assert_eq!(list1.local_value(), vec![400, 200, 300]);

}

```



You can find more examples in the examples and tests directories in the

crate repo.



## Using CRDTs



Ditto CRDTs are designed to mimic standard data type APIs as much

as possible. You can `insert` and `remove` list and map

elements, `replace` text elements, etc. Each edit generates an op

that can be sent to other sites for execution. When you execute an

op sent from another site, you receive a `LocalOp` that shows

exactly how the CRDT's value has changed.



The two complications of CRDTs that users have to worry about are:



  * How to send ops/state from one site to another

  * How to assign a site id to each site.



Ditto does not include a networking layer. However, you can find

info on how to send changes and assign site IDs in the docs below.



### Sending ops and state



CRDTs and ops are serializable with [Serde](https://serde.rs).

Serialization is tested against [`serde_json`](https://github.com/serde-rs/json)

(`JSON`) and [`rmp-serde`](https://github.com/3Hren/msgpack-rust)

(`MsgPack`) but may work with other formats as well.



Ops must be sent in the order they were generated. That is, if

a site performs edit A and then edit B, it must send op A before

it sends op B. State can be sent in any order.



Similarly, ops must be sent over a network that guarantees in-order

delivery. TCP fits this requirement, so any protocol sitting atop

TCP (HTTP, WebSockets, SMTP, XMPP, etc.) will work as a transport

layer for op-based replication. State can be sent via a protocol

that does not guarantee in-order delivery.



In general, when replicating a CRDT state you should send its

state struct, not the CRDT struct, because the CRDT struct includes

the site ID. For example, to replicate a `Json` CRDT you should

send the serialized `JsonState`, which can be created by calling

`json_crdt.state()`.



### Assigning Site IDs



A CRDT may be distributed across multiple *sites*. A site is

just a fancy distributed systems term for "client". Each

site that wishes to edit the CRDT must have a unique `u32`

identifier.



The site that creates the CRDT is automatically assigned to

ID 1. ***You*** are responsible for assigning all other sites;

Ditto will not do it for you.



Here are some strategies for assigning site identifiers:



* Reuse existing site identifiers (e.g. numeric client ids)

* Use a central server to assign site ids on a per-CRDT basis

* Use a consensus algorithm like Raft or Paxos to determine a new site's ID



Site IDs can be assigned lazily. If a site only needs read access

to a CRDT, it doesn't need a site ID. If a site without an ID edits

the CRDT, the CRDT will update locally but ops will be cached and

unavailable to the user. When the site receives an ID, that ID

will be retroactively applied to the site's edits, and the cached ops

will be returned to be sent over the network.



### Do I need a centralized server to maintain consistency?



CRDTs do not require a central server to ensure eventual

consistency; you can use them in peer-to-peer protocols,

client-server applications, federated services, or any other

environment. However you *do* need a way to assign unique site

identifiers, as explained in the section [Assigning Site IDs](#assigning-site-ids).

A centralized server is one way to do that, but not the only way.



A server may also be useful as an op cache for unavailable

clients. If you are using CRDTs in an application where sites are

often offline (for instance, a mobile phone app), you can use

the server to store ops and state changes until they have been

received by all sites.



### Duplicate ops



Ditto CRDTs are *idempotent* — executing an op twice

has no effect. As long as ops from a site are executed in

the order they were generated, the CRDT will maintain consistency.



### Sending ops vs. sending state



Usually, the most compact way to send a change between sites

is to send an *op*. An op is just a fancy distributed systems

term for "change". Each time you edit a CRDT locally, you generate

an op that can be sent to other sites and executed.



However, there may be times when it is faster and more compact

to send the whole CRDT state (e.g. if you're sending 100 or 1000

edits at once). You should replicate exclusively via state if you

cannot guarantee in-order op delivery.



### Other Notes



Collection CRDTs are inherently larger than their native equivalents

because each element must have a unique id. Overhead is most

significant when storing a collection of very small values — a `List`

will be many times larger than a `Vec`. If the collection itself

is immutable, you can significantly reduce overhead by switching from

a `List` or `Map` to a `Register>` or `Register>`.



## License



Ditto is licensed under either of



* [Apache License, Version 2.0](https://www.apache.org/licenses/LICENSE-2.0)

* [MIT license](https://opensource.org/licenses/MIT)



at your option.



### Contribution



Unless you explicitly state otherwise, any contribution intentionally

submitted for inclusion in Ditto by you, as defined in the Apache-2.0

license, shall be dual licensed as above, without any additional terms

or conditions.