Creating a custom global describe API using @RestResource

A colleague is working on a client that needs to know all the SObject names and all the field names within those SObjects. The Apex describe APIs provide this information but also a lot of other information that is not required in this case. So it is worth doing work at the server-side to cut down the information to only what is required by the client.

(In the org in question, the 300 SObjects produce JSON output of 800 kB, well below the 3 MB governor limit on HTTP responses.)

Salesforce’s @RestResource mechanism makes doing this pretty easy. The code below transfers the required information into instances of simple Apex classes, sorts the data based on label first then API name second, and then leaves it up to the platform to serialise those as JSON:

@RestResource(urlMapping='/v1/describe')
global with sharing class DescribeRest {

    global class Sob implements Comparable {
        
        public String sobLabel;
        public String sobApi;
        public Field[] sobFields;
        
        Sob(SObjectType t) {
            DescribeSObjectResult r = t.getDescribe();
            sobLabel = r.getLabel();
            sobApi = r.getName();
            sobFields = new Field[] {};
            for (SObjectField f : r.fields.getMap().values()) {
                sobFields.add(new Field(f));
            }
            sobFields.sort();
        }
        
        public Integer compareTo(Object o) {
            Sob that = (Sob) o;
            if (this.sobLabel < that.sobLabel) return -1;
            else if (this.sobLabel > that.sobLabel) return 1;
            else {
                if (this.sobApi < that.sobApi) return -1;
                else if (this.sobApi > that.sobApi) return 1;
                else return 0;
            }
        }
    }
    
    global class Field implements Comparable {
        
        public String label;
        public String api;
        
        Field(SObjectField f) {
            DescribeFieldResult r = f.getDescribe();
            label = r.getLabel();
            api = r.getName();
        }
        
        public Integer compareTo(Object o) {
            Field that = (Field) o;
            if (this.label < that.label) return -1;
            else if (this.label > that.label) return 1;
            else {
                if (this.api < that.api) return -1;
                else if (this.api > that.api) return 1;
                else return 0;
            }
        }
    }
    
    @HttpGet
    global static Sob[] get() {
        
        Sob[] sobs = new Sob[] {};
        for (SObjectType t : Schema.GetGlobalDescribe().values()) {
            sobs.add(new Sob(t));
        }
        sobs.sort();
        
        return sobs;
    }
}

When accessed using /services/apexrest/cveep/v1/describe.json, this produces JSON (formatted here to better illustrate the structure) taking about 10ms per object at the server-side:

[
    {
        "sobLabel":"Absence",
        "sobFields":[
            {"label":"Absence","api":"Name"},
            {"label":"Absence Type","api":"Type__c"},
            {"label":"Claim","api":"Claim__c"},
            ...
        ],
        "sobApi":"Absence__c"
    },
    ....
]

Now that the governor limits have been removed on describe calls the first limit that will be hit is probably the 3 MB response size limit.