set arrary data to Avro array type with C language

Question

I am programming in Objective-C. I am using Apache Avro for data serialization.

My avro schema is this:

{
"name": "School",
"type":"record",
"fields":[
  {
   "name":"Employees",
   "type":["null", {"type": "array",  
                    "items":{
                       "name":"Teacher",
                       "type":"record",
                       "fields":[
                          {"name":"name", "type":"string"}
                          {"name":"age", "type":"int"}
                       ]
                    }
                  }
          ],
   "default":null
  }
] 
}

In my Objective-C code, I have an Array of Teacher objects, each teacher object contains value of name & age.

I want to write the teacher array data to file using Avro with the schema showing above. I am mainly concern about how to write data to the Employees array defined in above schema.

Here is my code (I have to use C style code to do it, I follow the Avro C documentation):

// I don't show this function, it constructs the a `avro_value_t` based on the schema. No problem here.
avro_value_t school = [self constructSchoolValueForSchema];

// get "Employees" field
avro_value_t employees;
avro_value_get_by_name(school, "employees", &employees, 0);

int idx = 0;
for (Teacher *teacher in teacherArray) {
   // get name and age
   NSString *name = teacher.name;
   int age = teacher.age;

   // set value to avro data type.
    // here 'unionField' is the field of 'Employees', it is a Avro union type which is either null or an array as defined in schema above
   avro_value_t field, unionField;
   avro_value_set_branch(&employees, 1, &unionField);
   // based on documentation, I should use 'avro_value_append'
   avro_value_append(&employees, name, idx);
   // I get confused here!!!! 
   // in above line of code, I append 'name' to 'employees', 
   //which looks not correct, 
   //  because the 'Employees' array is an array of 'Teacher', not arrary of 'name' 
   // What is the correct way to add teacher to 'employees' ? 

   idx ++;
}

The question I want to ask is actually in the code comment above.

I am following that Avro C documentation, but I get lost how can I add each teacher to employees ? In my above code, I only added each teacher's name to the employees array.

Answer 1

I think there are two things wrong with your code, but I am not familiar with Avro, so I can't guarantee one of them. I just quickly peeked at the documentation you linked and here's how I understood avro_value_append:

It creates a new element, i.e. Teacher and returns that via the reference in the second parameter (so it returns-by-reference). My guess is that you then need to use the other avro... methods to fill that element (i.e. set the teacher's name and so forth). In the end, do this:

avro_value_t teacher;
size_t idx;
avro_value_append(&employees, &teacher, &idx); // note that idx is also returned by reference and now contains the new elements index

I'm not sure if you set up employees correctly, btw, I didn't have the time to look into that.

The second problem will arise with your usage of name at some point. I assume Avro expects C strings, but you're using an NSString here. You'll have to use the getCString:maxLength:encoding: method on it to fill a prepared buffer to create a C string that you can pass around within Avro. You can probably also use UTF8String, but read up its documentation: You'll likely have to copy the memory (memcpy shenanigans), otherwise your Avro container will get its data swiped away from under its feet.