Allow change of parallelUploads config in dropzone

[mikaldl]

Description

When running SilverStripe in a LoadBalancer multi-server setup, the parallel uploads causes problems with unique key constraints due to the fast distribution of requests between different servers and one database connection.

With parallel uploads already being available in the dropzone module, it can be handy to be able to change this to prevent earlier said problems.

Additional context or points of discussion

To sketch a good picture in how this occurs with the setup I am currently running for a client:

• WEB-01, WEB-02 as dedicated CMS/Frontend servers
• LB-01 for load balancing all the requests
• DB-01 (with a slave setup)

We are seeing every /upload request being distributed towards WEB-01 and WEB-02 simultaneously causing request 1 being correctly saved, but request 2 fail because of the duplicate key constraints.

Validations

• You intend to implement the feature yourself
• You have read the contributing guide
• You strongly believe this feature should be in core, rather than being its own community module
• You have checked for existing issues or pull requests related to this feature (and didn't find any)

PRs

• NEW Allow changing parallelUploads config in UploadField #1539

[emteknetnz]

I haven't looked into this very deeply though based on this answer it may be that we want to make it so that uploadMultiple is configurable to false instead

[mikaldl]

The package uses version 6.0.0-beta.2 of Dropzone. uploadMultiple would mean there will be multiple files inside one request, which is different compared to parallelUploads which result in two parallel requests. Changing the uploadMultiple to true would probably break the PHP upload field because of different behaviour.

More information about available options is found here: https://github.com/dropzone/dropzone/blob/v6.0.0-beta.2/src/options.js#L35

[emteknetnz]

Linked PR has been merged, it will be released as part of the upcoming CMS 5.4.0 release