锁定文件以更新一部分数据或所有文件内容

Question

I’ve file which can be updated from several of process, hence I want to use locking and found https://github.com/gofrs/flock which might helps

But I think that issue is a bit more complicated , for example to update the file the api needs to read the file/ section before (we are providing api to read the file or the application objects by name )
Get the json data modify it in the memory and update the file.

There is two options:

1. updating the all file content
2. updating section on the file i.e. one application properties

The problem is like following:

 1. Process A & Process B (can be more…) Reads the object  of
    application name  `node1`

 2. Process A update the section (node1) with new data (for example
        change the kind property and update)

 3. Process B want to do the same , the problem is that between the time
        it reads the data and the time it wants to update ,the data is not
        valid since other process already update it.

in addition the same scenario is valid to all the file context

Race condition issue…

This is a short example of the file that some stateless process can update at any given time

ID: demo
version: 0.0.1

applications:
 - name: node1
   kind: html5
   path: node1
   expose:
    - name: node_api
      properties:
         url: ${app-url}


 - name: node2
   kind: nodejs
   path: node2
   parameters:
      quota: 256M
      memory: 256M

How we can overcome this issue, or maybe simplify it to avoid race condition and collusion?

Answer 1

This is a common problem so look for known solutions. For example, optimistic locking.

Something like this pseudocode:

lock file for read
read file into data1
release file lock
hash data1 as hash1
update data1
lock file for update
read file into data2
hash data2 as hash2
if hash1 != hash2
    release file lock
    return error
write file from (updated) data1
release file lock
return success